Borderlands 2

Awesomnauts !!! really awesome game, might take a bit of learning against bots first but once you get a hang of it that game is a joy !

If you like platformers: Rayman Origins.

shank 2

If you like shadowgrounds, then you'll probably like Nation Red. And Shank 2 Survival Mode

Trine , Street fighter , Shank 1

Binding of Isaac.... eventually.

Also Castle Crashers is great MP fun

EDIT: Shank 2 is amazing with friends.

LEGO games.

Jamestown - with 4 Player local Multiplayer (Mouse, Keyboard, Joypad)

I really like this game because of the fun to play, the awesome soundtrack and the coop!

This game was available in Humble Indie Bundle 6

This site

Little Fighter

Trine 1 and 2, Jamestown, Serious Sam 3: BFE, Shank 2

Trine 1 and 2 - are awesome in local coop.

Lara Croft and Guardian of light - really nice if you like isometric view

Shank 1 and 2 - is beat em up (I still need to find some time to play Shank 2, first one had nice coop).

Worms - old ones were really nice for competitive local multiplayer if you are looking for it.

Rayman Origins - people say it's amazing, I know demo was but I still didn't get it.

There are probably many more games that could be interesting.

Dungeonland seems pretty good, It's gonna be released on the 29th, if you preorder you get some nice extras.

If you like kart-racing games, F1 Race Stars has a splitscreen option, even in campaign mode. Played it at a friends house on his PC and it worked like a charm. There's a demo you can check out in the Steam store.

Trine 1&2, like mentioned above.

Portal 2 is amazingballs and does indeed support local co-op on the PC.

Suchanevillink~

local co-op? you dont know jack is great fun if you have someone to play it with :)

Awesomenauts, and soon Dungeonland will be released (which seems great for Local co-op).

Settlers 2 Gold Edition. Split screen with two mice support. One of my favorites from the past.

dungeon defenders

Dungeon Defenders/Sanctum!

Shogun 2 Total War

Well it sounds like you're looking for PC games, but Halo and Gears of War on 360 have great co-op.

Serious Sam 3 also has local co-op. And I think I saw in a recent update that Portal 2 on PC now has local co-op? (surprised it didn't have it before)