What is a fun, multiplayer game I can get for 15- 20$

nothing.

What kind? FPS? Strategy? RPG? Etc.

Payday's on sale right now, that's not bad. L4D2 could possibly be the next community deal too, and that's a good choice also.

L4D2 is fun.

Blacklight. Oh wait.

Portal 2 :D

Do you have Borderlands?

Sanctum? Magicka?

Or pay nothing and play Tribes: Ascend or Team Fortress 2.

Unreal Tournament! 99 is the best imo, 2004 is fun with vehicles, UT3 didn't add anything to the series so I wouldn't bother with that one.

Wait a bit and you can get L4D2 for 5 bucks.

Try earth defense force: insect Armageddon

Killing Floor, which will be probably discounted at 75%.

Frozen Synapse is really fun.

I've been really enjoying Gotham City Impostors on my 360, so I can only assume it would be fun on PC as well.

dayz

Brink, oh wait

Get Left 4 Dead 2. It's worthy!

and you can you another copy for giveaway ;)

No one seems to have offered Payday™ the Heist yet…

Dungeon Defenders. Amazing fun IF you have friends. Not so much without them.

Blacklight is really good, but it's free. I like it better than most games I pay for. TF2 too.

counter strike source

http://steamcommunity.com/id/MikeOMe/games/

Trine 2, Portal 2, All zombies must Die =]