Some have called it just another Flash game, but it's more than that.
It's a damn good Flash game. Like... Really. Damn. Good.
It's a top down shooter with a lot of upgrades and weapons and stuff, just damn good.

A simple fun zombie FPS. Think Counter-Strike but with zombies.
You have a bunch of waves, each wave harder zombies show up.
In between waves you purchase weapons from the trader.
There are different classes which give you different perks for different guns.
The perks can also be leveled up, but leveling is a bit of a grind.
The graphics are nice, a bit rough around the edges though, and zombies make a nice squishy sound when they die.

I've spent hundreds of hours playing this amazing game, and it's updated almost every month in case it starts to get stale. TF2 has definitely changed over the years, but I believe it's for the best.
If you're new to TF2 make sure you do the training first or you'll probably get owned by the regulars out there.