ThiefNet is a website that the Cooper Gang uses to sell their stolen loot for coins, and then use those coins to buy new Power-ups for the gang to use in the field. It also has a chat room, in which Bentley meets Penelope. 
Sly 2: Band of Thieves
First introduced after completing the mission Bug Dimitri's Office, this in-game online website allows you to buy a vast amount of power-ups. When you pickpocket a guard that has a shiny pocket, you will get loot. You can go back to your Safehouse and sell that treasure for a set amount of coins. (Can vary from different treasures like a Bronze Comb).
Sly 3: Honor Among Thieves
In Sly 3: Honor Among Thieves, ThiefNet returned with several other features in the game. A new feature was added with stealing loot, which automatically gives you coins after you steal the loot. (Stealing a Sapphire will result in 91 coins added to your coin collection.) There is also a chat room which Bentley used to contact with Penelope.
Sly Cooper: Thieves in Time
ThiefNet also appears in Sly Cooper: Thieves in Time, this time with a big change. New power-ups are available and the layout is different. Power-ups can be bought for all of the Cooper Gang (Excluding ancestors). Unlike previous games, bought power-ups are still displayed on the list of power-ups.
- On the left side of Thieves in Time's ThiefNet, you can see Bentley's System Cracker, Bentley's Alter Ego, and the Spark Runner, along with their version numbers.