Multiplayer games for a Potato Tier computer?

Is it required to be Steam, and are multiple gamepads an option?

Local co-op allowed, or just online?

TF2 :D

Terraria; Hammerwatch; Monaco: What's Yours Is Mine

They will probably run well.

garry's mod would do

Minecraft? Runescape..., Realm of the Mad God.., Terraria/Wanderlust..I think this might even work for you for a newer title: http://store.steampowered.com/app/227780/ and similar indies/early access.

Counter strike 1.6? or fistfull of frags?=> is a free to play mod of half life 2 recently launched in steam.(cowboys everywhere)

Heroes of Might and Magic 3. Best hotseat game of alltime.

TF2, League of Legends, WoW, Path of Exile, Diablo 3...wait, I should probably ask what you consider decent fps. I consider 30 decent, but that's probably because I spent the last 7 years with a laptop that was running a ATI Radeon Mobility X1200 GPU. Yeah, your card is godlike in comparison.

Oh, and I pretty much only had a minor upgrade to an Intel HD 3000 card wielding laptop recently.

Try a 'Worms' Game or a 'Counter-Strike'. :) Or Aerena, Terraria, Monaco, Stronghold Crusader.. there are many popular Indie Games and old jewels. :) Oh 'Killing Floor' is a good idea, too. The Perfect World MMORPG's were okay, too.

realm of mad god. a 2002 flip phone can play that game

King Arthur's Gold

Rainbow Six Vegas 2, and find an mp crack from TeknoGods. It has multiplayer, and co-op ^^

Little Fighter 2
It's an awesome free game.
I used to spend my vacation playing the crap out of that game (+ mods) xD

Upto 4 players can play from one keyboard.. you can put your hands on it..
Allowed network of 2 computers.. 4 players can play from each.

CS 1.6

Transformice
What's wrong with a potato tier computer? That's enough power to run GLaDOS. :p

okay, the original starcraft, not on steam though.

Aside that, have you tried chess?

Fistful of Frags

Battle for Wesnoth!

Doom, quake, original UT...

Warzone 2100

Garrys mod ran good for me when I use to play it with a Geforce 7300 gt

Even if you can't run Team Fortress 2, there's always Gang Garrison.

Spiral Knights

MMORPG Lineage 2 or Wow
FPS: Counter Strike Source or CS 1.6, Team Fortress 2, Battlefield 2, Call of duty modern warfare
RTS: Sins of a solar empire. Red Alert 3, World in Conflict

Guild Wars Trilogy + Eye of the North Expansion

Not much of an "MMO"RPG (these days) but the npc heroes that you control and customize can get you through the game much better (depending on your handling) than most of the players. Still plenty of people playing though for the MMO feeling.

Silly that they still sell Eye of the North separate but its a must buy if you like the game. And its all just 1 game, each "campaign" is a separate continent that you seamlessly travel between.