Update 2: As some users have noticed, there are a few bugs in the new unlockable system. Valve's Robin Walker explains to
RPS:
"As in everything else we do, we want to iterate, so this is really the first step towards our desired implementation," said Walker. "We also think we have bugs in the system as well, since we're seeing some players not finding any items after a few hours of play, which is not at all how it's tuned. We want unlockable drops to be a pretty regular occurrence for players, with the cosmetic hats being the rare finds."
Update: TF2 steward Robin Walker has confirmed to us that there is now a new way to earn unlockable weapons in the game.

TF2 unlockables can now be earned outside of achievements. Hats off to you, Valve.
"Items now drop randomly, in addition to the old (pre Spy/Sniper unlockables) achievement tied weapons," said Walker. "We're going to release info on the new system on the blog tomorrow."
Walker also noted that a series of head-slot hat items (see above) can now be earned in the same fashion.
Original story: The largest Team Fortress 2 update to date has finally been released by developer Valve, kick-starting a four-day free trial for the game.
Valve also notes that the title will be 50% off its regular price on Steam for the duration of the... Read more