Team Fortress 2 has been updated many times by Valves, and part of those updates has been the addition of new weapons and achievements for each class. However, these unlocks are not very well documented - which is why I've complied this list of all the unlockables and how to get them.
Updates Ahoy!
One of the best things about Valve, besides the great games they make, is that they support their games well beyond release. Team Fortress 2, which was released two years ago, would by now have been basically abandoned by most other developers. But Valve has continued to squash bugs with regular patches. They've also kept TF2 fresh by up by unleashing balance updates and new unloackable weapons which enhance and change how some classes play.
Currently, there are unlockable weapons and items available for the Medic, Scout, Pyro, and Heavy classes. These unlockables are earned through achivements associated with each class. When you complete a certain number of achievements, you unlock an item.
Medic Unlocks
Blutsaugher
Once you have earned 10 of all the achievements for the Medic, you'll unlock the Blutsaugher. This is a new syringe gun which does not score critical hits, but it transfers a bit of the health from each hit against an enemy to the medic. It is not an exceptional weapon, which makes sense, at it would be bad to give the Medic too much offensive power. It does give the Medic a little bit of extra health, however, making it much easier to beat a fighting retreat. There is little reason not to use this once you earn it.
Kritzcrieg
Once you have 16 achievements, you'll earn the Kritzcrieg. This is replaces the normal medigun with one that, rather than providing invulnerablity as the ubercharge special, provides a 100% chance to perform a critical hit. This can obvious be devestating to a front line, although it is probably not as useful against defense positions that are defended by well-placed sentries. They are too deadly, and will kill whoever is ubered well before they can do much damage, unless you uber a Demo-man who is extremely good at what he does.
Ubersaw
Earned when 20 achievements are unlocked, the Ubersaw is a replacement melee weapon which, unlike the regular saw, charges your ubercharge! This is less useful than you might think, because come on, how often is a Medic in melee range? Usually, it is only a moment before the Medic is stabbed in the face by a spy. That said, if you do manage to use it, then it is extremely handy. And besides, there's no real reason to carry the normal saw after you have this.
The Pyro
The Flare Gun
Unlocked when 10 achievements are completed, the Flare Gun replaces the Shotgun and acts as a long-range utility weapon. It does little direct damage, but sets the target on fire, doing a fair amount of damage over time. The damage done is not enough to kill anyone outright, but unless a medic is readily available, it will probably send them fleeing. The downside is since it replaces the Shotgun, you're left without a good mid-range weapon.
The Backburner
Earned at the 15 achievement mark, the Backburner is a flamethrower which looses compression blast and gains a 100% chance to critcal when hitting a target from behind. It also grants the Pyro an extra 50 health points. It is an extremely useful weapon which makes the Pyro a close-range ambush specialist, and even if you're not into surprising people, it is still often useful. Losing compression blast sucks when trying to seperate a Medic from a heavy, though.
The Axtinguisher
Earned when 22 achievements are unlocked, the Axtinguisher performs a critical hit against an enemy that is on fire. In practice, this is often a one-hit kill. It is weaker against enemies who are not on fire. This is a very useful weapon when you have an immobile or low-threat enemy like an Engineer or Medic backed into a corner, or when you ambush an enemy.