With the "Uber Update" on the way, Valve has opted to switch to a micro-transaction driven model for their popular team shooter.
If you haven't yet had the chance to play Team Fortress 2, you really don't have an excuse now. Valve is is making the popular shooter permanently free-to-play; the only form of monetization will be through micro-transactions.

"We've been toying with the idea of making Team Fortress free-to-play ever since the Mann-conomy update [in September 2010]," Valve's Robin Walker told Develop.
According to Walker, Valve has no plans to implement a premium subscription model or "pay-to-win" options. And despite the new focus on micro-transactions, free in-game items will continue to be available.
Valve's main goal in making Team Fortress 2 free is to get as many people as possible playing it online. It's the end result of months of experimenting with the price, which saw it dip as low as $2.49 at some points.
While Valve believes Team Fortress 2 will be more successful as a free product though, they caution that the same might not be true for their other games.
"It seems dangerous to assume that [free-to-play would work] for all our products," Walker said.
Team Fortress 2 was previously in the midst of a free week that was to include the release of their massive "Uber Update." Look for the update later tonight.
Source: Daily News from GamePro.com