Team Fortress 2

Taking a leaf out of Company of Heroes Onlines' book, TF2 will let you exchange cash for ingame items. Micro-transactions are getting more and more popular :-(

Quote from KOTAKU:

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No longer will you have to endure hours of Team Fortress 2 play just to unlock the game's hidden weapons, defensive items and wearable accessories. Today, Valve opened the Mann Co. Store, which lets you buy goods with real-world cash. The update not only introduces a way to buy new guns, hats and fish-wrapped-in-newspaper melee weapons, it includes the long-promised Polycount Pack update. That's the community created item sets, which add 17 new items for five of the game's classes. Players can blow real-world cash on those items starting today. Prices range from 49 cents to $4.99 USD. Funds can be deposited to the newly launched Steam Wallet. What's not in this update? The new game mode that Valve had previously promised. While Valve stands to profit from selling in-game items, so does the community. The in-game store FAQ notes "community contributors will receive a percentage of sales on items they've created." The Mann Co. Store may be goods news for the lazy, but the miserly needn't be overly concerned about the change. Valve notes in its new Mann Co. Store FAQ that the Polycount Pack items are unlockable and craftable through normal means, just like other items. Read more about the new "Mann-conomy" in the service's newly released FAQ and peruse the new item sets at the Team Fortress 2 official site to see if that five dollar Spy knife is something you'd like to buy.

Sources:
Kotaku
TeamFortress.com