Fat Princess

Fat Princess takes 32 players, 5 classes (each with 2 forms), 4 multiplayer modes, a dash of whimsy and a pinch of customization, beats it until smooth and applies it liberally to your free time.  In short, Fat Princess is a fun time-suck that will leave you wanting more after each round.
 
The game is obviously influenced by the likes of Team Fortress 2, but instead of a first person shooter, we get a 3rd person perspective, heavily strategic game that forces you to work as a team in order to succeed.  The game achieves this by really giving no class more power over any other class, therefore one class can't just go in and wreck entire teams.  It also limits the abilities of each class by giving each of the classes 2 separate forms.  If you want to freeze people as a mage, you have to give up your flames in order to do so, and even though you can swap from 1 class form to the next on the fly, most attacks will need to be charged in order to get the most out of them, and you lose your charge if you swap from one form to the next.  Each class's alternative form must be unlocked by upgrading its respective hat machine back in your team's base.
 
The graphics are where the whimsy comes in.  It is a very light hearted looking game that doesn't take itself too serious.  Its focus is on fun first and seriousness second.  The game is meant to appeal to a wide audience and does so by mixing childlike graphics with visceral combat and gore (which can be disabled if you must).  The sounds are great too and add to this whimsy.  You will often hear many popular gamer/geek quotes (cliches) from the announcer:  "They're in our base, killing our dudes!"  is one I can think of, and the Princess once called someone who died infront of her a "Noob!" which was pretty awesome I have to admit. The combat sounds match the graphics well and many of them have a very over-the-top cartoon vibe to them.
 
The gameplay is simple yet fun, and leaves a lot of room open to really learn a lot about each class's particular skills.  But mastering a particular class's skills isn't nearly as important as master teamwork.  In Fat Princess teamwork is more important than almost any other game, and I mean that in terms of public games.  I can not stress this enough, but without even a modicum of teamwork, you and your team will fail.  There is no question about it, it is impossible for any 1 player to adequately influence the outcome of a single game (unless you're playing with just bots).  You must work with teammates and I highly recommend that you buy a mic if you think you will be playing this game for any serious amount of time.
 
The 4 modes offer enough variety to keep this game relevant for quite some time to come.  Each feels different from one another with the exception of Team Deathmatch which feels like the other other modes, except stripped from any objectives... although it does require different tact if you truly want to win, like Arena mode in TF2.  However the most popular mode is most certainly Rescue the Princess which is really just capture the flag with some new twists.
 
The game is definitely worth its asking price and you can just tell, from the first time you play Fat Princess that it is going to be around for awhile.  This won't be like a good many console downloadable games where the game is popular one week online then the next no one can be found playing it.  Fat Princess will hold the public's interest for quite some time and at $15 you'd have to hate cake to not buy this game.