A good, fresh alternative. Give it a shot.
Anyway: Monday Night Combat (MNC). A frantic little shooter that takes concepts from tower defense and melds it with the whoosh bang wahay of online shooters. Spawn from your side of the map with bots and try to get to the other side and destroy the 'Money Ball' (really just a great big target to drive the action but anyway). Maps are fantastically well designed so there's usually little delay between spawn and shoot frenetically. Multi level action is well implemented and advantageous enough to encourage use, but not so critical that those not familiar with the level can't have fun. And the classes are interesting and varied, not to mention the various individual skills available to each lends itself to allow real depth of play.
But blah blah blah is it as good as TF2? Well, as I said above (weren't you paying attention!?) it really is quite different, especially in play style. But if pressed I would answer with this: "A year ago no, but now yes". The move away from the special guns, constant tweaks, ugh-hats-ugh, towards a more pure "everyone has mostly the same stuff, just use it" means that winning a match is based on skill and experience, not items or other faff.
So to use a ridiculous metaphor: MNC is a delicious plain ice cream cone to TF2's overblown, overcrowded, overdecorated banana split mess. Yes TF2 is still has some good stuff in it somewhere and I'm all for banana split, but for god's sake sometimes I just want some ice cream.