A new update
to Valve's Steam game service today introduces the Steam Wallet, allowing users to add money to their accounts for purchasing in-game items or full games. Team Fortress 2 will feature the "first of many" store services, although what games might follow have not been hinted at. Valve has stated that the Wallet will not require players to purchase bundle amounts as with other online game platforms, and that stored money will not be converted from their real-world equivalents (USD, GBP, Euros).
The TF2 update that accompanies the Steam Wallet launches the in-game Mann Co. Store, which allows players to buy cosmetic upgrades such as new hats, new weapon skins, and community-made content. Valve states that the purchasable items will not affect gameplay or give any players an edge for buying content. Existing weapons currently held by players will now feature a "vintage" tag, indicating that they were obtained the old-fashioned way by simply playing the game. Trading items, a feature long requested, has also been enabled as has the option to gift purchases to a friend. For any other questions Valve has released a "Mannconomy FAQ" available here