The surviving correspondence of Hugh Despenser the Younger in the 1320s, when he was at the height of his power, makes clear that he had spies and informants everywhere, and encouraged people to snitch on their neighbours to him.
A man named Thomas of Bishopstone ('Bysschopeston') lived in the village of Bishopstone on the Sussex coast in the 1320s. One Roger Lumbard, who apparently came from Lombardy in northern Italy, or at least his ancestors did, knew Thomas and was perhaps a neighbour, and told Hugh Despenser the Younger that Thomas had made a passage along a cliff by the sea to enable Edward II's enemies to flee abroad. Furthermore, Lumbard claimed, Thomas was an enemy of the king himself and an adherent of Adam Orleton, bishop of Hereford, with whom Edward and Hugh were feuding at the time, and was smuggling Bishop Adam's letters overseas. A royal order to arrest Thomas of Bishopstone was issued on 1 June 1324, and the unfortunate man was forced to acknowledge a debt of £100 to Hugh Despenser to clear his name and to be able to return to his home. Edward II and Hugh Despenser were in the village of Bishopstone on 31 August 1324, which is hardly likely to be a coincidence, especially as Edward never set foot there at any other time in his reign. Thomas of Bishopstone stated that one Simon Croiser was the man sent to arrest him, which is true; the order of 1 June 1324, recorded on the Patent Roll, was indeed given to Croiser. 
Roger Lumbard is the only informant of Hugh Despenser whose name I've been able to discover. This is thanks to his victim Thomas of Bishopstone, who sent a detailed petition to Chancery setting out what had been done to him. The petition was written in Anglo-Norman by clerks though Thomas surely described his ordeal in English, and the angry Thomas called Roger Lumbard a lapin, which from the context appears to mean 'scoundrel' or 'rogue' (in modern French it means 'rabbit'). It's not clear whether Lumbard already worked for Hugh Despenser, or just knew that Hugh encouraged snitching and hoped to receive a reward from him for telling tales about Thomas, whether his allegations were true or not.
|Part of Thomas of Bishopstone's petition.|
At Easter 1324, Hugh Despenser sent a letter (which fortuitously still exists in the National Archives) to Sir John Botetourt, a man in his sixties who held the Oxfordshire manor of Iselhampstead with his wife Maud, who had inherited it. Hugh demanded that the manor be handed over to him, and told Botetourt that as he had "received" his son John the younger, a Contrariant of 1321/22 and an adherent of the executed Contrariant Bartholomew, Lord Badlesmere, the king "can hang and draw you" (il vous peut pendre e trayner). John Botetourt, unsurprisingly, did hand over Iselhampstead to Hugh Despenser a few days after receiving this threat, and the manor was in Hugh's possession at the time of his downfall in October/November 1326. This whole affair reveals firstly that Hugh was willing to threaten people with execution unless they gave him their manor(s), and secondly, that someone must have told him that John Botetourt's son had visited him.  As with the case of Thomas of Bishopstone just a few weeks later, it seems that Hugh seized on information his spies sent to him as a pretext to seize a manor or two or to demand a large fine; he used people's adherence, real or imagined, to Edward II's enemies to enrich himself at their expense. Another common tactic used by Hugh and his father Hugh the Elder between 1322 and 1326 was to accuse men of having supported Thomas, earl of Lancaster in 1321/22, and to demand a heavy fine or to take one or several of their manors as punishment.
Hugh sent a letter on 5 October 1324 to the French nobleman Henri, Lord Sully, butler of France, who had been in England with Edward II in 1322 and who was briefly captured by Robert Bruce, king of Scotland, at the battle of Byland in October that year. The letter began with Hugh expressing his astonishment that Henri had sent a messenger to England bearing his letters, but had not sent any to Edward II or to Hugh himself (nous nous merveilloms molt qe vous envoiastes un garceon en Engleterre ove auscunes lettres ... et ne envoiastes nulles a mon seignur le roi
Dengleterre ne a nous; Hugh often kept the drafts of his own letters and the crossing-out is his scribe's).  And how did Hugh know that Lord Sully had sent letters to England even though neither he nor the king received any of them? Obviously, someone informed him. Hugh addressed Sully as his "very faithful friend" (tresfiable ami), which he possibly intended sarcastically, and seemed keen to ensure that Sully was aware that little in England escaped him. Unfortunately, Hugh didn't clarify who did receive Henri Sully's letters, but might have had informants at various ports telling him who sent messengers into England, and perhaps employed spies in the households of other nobles.
Hugh had informants not only in England, but in Scotland too. In a letter to his cousin, ally and perhaps friend Sir Ralph Basset of Drayton (d. 1343) in early October 1324, Hugh stated that he had several "confidants in those parts who have talked to us" regarding a potential meeting between Edward II and representatives of Robert Bruce, king of Scotland, around the octave of St Martin, i.e. c. 18 November.  Hugh trusted Ralph Basset, and frequently addressed him as "dearest cousin", "fair cousin" and "beloved cousin". He often told Basset things which he ordered him to keep secret, including this planned meeting between Edward and Scottish envoys, and his letters to him are very illuminating.
Another man Hugh Despenser confided in was Sir John Sturmy, admiral of Edward II's eastern fleet and someone who often appears in Edward's chamber accounts. Hugh told Sturmy in October 1324 that he had an informant on the Continent with Roger Mortimer of Wigmore and his allies, the other Contrariants who had escaped from England in 1322/23, who was sending him information about the men and their movements. Hugh added that he could not put these things into writing, and understandably but unfortunately for posterity did not identify his informant, whom he called "of their faction" (de leur covigne). This same letter indicates that he knew, two years in advance, that the English exiles intended to land in Norfolk or Suffolk with a large armed force, with the aid of the king of Bohemia and the count of Hainault.  Ralph Basset of Drayton, on the Continent, also kept Hugh and Edward II informed of the exiles' movements, and on 6 December 1323 stated that his own spy had told him they were on their way to Germany. Basset stated openly to Edward II that he had instructions - presumably from Hugh Despenser - to spy on them. 
The year 1324, when Hugh Despenser the Younger was directing Edward II's rather brief war against France, the War of Saint-Sardos, and when much of his correspondence survives, would appear to be the year when he reached the zenith of his power and his willingness to commit blackmail and extortion. It was also the year, for example, when he and his father imprisoned Elizabeth Comyn, great-niece of both King Henry III of England and King John Balliol of Scotland, in Surrey until she handed over her most valuable manors. Either in 1323 or 1324, Hugh the Younger imprisoned Sir John Inge, formerly sheriff of Glamorgan and a man who had given him years of faithful service, after he took against Inge for some reason. Hugh's last known letter to Inge, dated c. October 1322, cheerily told him "we are very worried about having some reason for which we might be prepared to harm you in some way", and he did find a reason: he imprisoned John Inge and members of his council in Southwark because of his "rancour towards him" and his "anger towards him". Hugh made Inge and six guarantors promise to pay him a ransom of £300 for Inge's release, and Inge's councillor Thomas Langdon died in Hugh's prison.  Various contemporary chroniclers stated that even the great English magnates were frightened of Hugh Despenser the Younger, and it's really not hard to see why.
1) Calendar of Patent Rolls 1321-24, p. 423; The National Archives SC 8/17/841; Calendar of Close Rolls 1323-27, p. 217.
2) TNA SC 1/37/5; Calendar of Fine Rolls 1319-27, p. 423; CFR 1327-27, p. 53; Feet of Fines, CP 25/1/19/74, no. 5.
3) The War of Saint-Sardos (1323-1325): Gascon Correspondence and Diplomatic Documents, ed. Pierre Chaplais, pp. 79-80.
4) War of Saint-Sardos, pp. 75-7.
5) War of Saint-Sardos, pp. 72-3.
6) War of Saint-Sardos, pp. 2, 5.
7) Cartae et Alia Munimenta quae ad Domimium Glamorgancia Pertinent, vol. 3, pp. 1101-4; TNA SC 8/176/8753, SC 8/59/2947; CPR 1330-34, p. 404.
My book about Hugh, Hugh Despenser the Younger and Edward II: Downfall of a King's Favourite, and my article ''We Might be Prepared to Harm You': An Investigation into Some of the Extortions of Hugh Despenser the Younger', Journal of the Mortimer History Society, 2 (2018), pp. 55-69.