"Previously, we rolled randomly at intervals to see if you got an item drop," Valve explains. "Now we roll to determine when your next item drop will occur. So you're guaranteed to find items at fairly regular intervals." The drop rate has also increased "compared to the previous average interval." It all means more hats for you.
The drop tweaks also make 'idle servers'--servers which simulate an active game, allowing players to get items without even being present--less fruitful, as there is now a cap to the amount of time that will produce items each week.
Valve added the first round of fan-created items to Team Fortress 2 last month, including a new cosmetic hat for each class and several new weapons. You can also wear hyperkinetic rabbity thing Max's decapitated head if you buy Sam & Max: The Devil's Playhouse, not to mention wield his pistol and Sam's revolver.