Support. In TF2 I'm always the medic until it's shown that something else is needed, and in L4D I tend to grab the sniper rifle and do a lot of hiding and pistol shooting. I'm not a good sniper, but for some reason it comes naturally for me in L4D: I can headshot a hunter off of somebody at any range, and quickly enough that the damage animations glitch and blood is flying everywhere around the location that the pounce took place. Always good for a laugh, but mostly they just appreciate the instant rescues from a team player who pays attention.
Then there was my Monk (healer) in Guild Wars, Monk/Assassin; I had teleporting and the assassin defense skills along with low-mana-cost healing, a stamana-healer. What really made me good however, was my willingness to NOT HEAL somebody because they were being stupid. If a foolish tank is soaking up DPS because he doesn't know what a defense skill is, then my healing is better spread over the 6 other people whom I CAN save. Most monks complain about not being thanked for being a healer, I got thanked every time, because I was good at it.
This one time x3 it was so awesome, was a 16 player room and we had just finished a 4 wave massacre of countless enemies, and for some reason all the healers decided to broadcast their remaining mana (via ctrl+clicking the mana bar I believe) so I did the same...
Healer1: 2/40
Healer2: 6/50
Meeeeee: 42/50
And it wasn't because I wasn't healing anybody either. Stamana build *thumbsup* Lots of signets.