Renegade Ops

rayman:origins is excellent, supporting 4 players local co-op. I've spent lots of time on it with my gf.
edit: rocketbirds: hard boiled chicken is another good local co-op game and I believe that only shank 1 has local co-op option. shank 2 supports the online co-op.

A Virus Named TOM.

Joint Ops

crazy frog racer 1 and 2 ;p and marvel ultimate alliance 4 coop ;p

Insanely Twisted Shadow Planet.

Also, It's only on PSN and XBLA but Scott Pilgrim vs. the World is basically River City Ransom.

cross off resident evil 5 it doesn't have local co op

Mortal Kombat Kollection

Edit: by the way, are we only talking about PC games?

Ones that I didn't see in the OP:
R.A.W. Realms of Ancient War;
Dungeons and Dragons: Daggerdale;
Orcs Must Die 2;
Lord Of The Rings: War in the North;
Sacred 2 (has it on the Xbox... not sure about PC).

Add The Last Crusade, i forgot that on my last list ;D
and Resident Evil 5 HAS coop
( http://z6.invisionfree.com/Resident_Evil_4_PC/index.php?showtopic=16219 )

video : http://www.youtube.com/watch?v=UWyUyOO3uzw

I think at least these hasn't been mentioned before:

Bang Bang Racing - split-screen
Bionic Commando: Rearmed - local co-op
Blur - split-screen (NOTE: 1 person may have to use keyboard/mouse)
Ghostbusters: Sanctum of Slime - local co-op
Overlord - split-screen (NOTE: 1 person may have to use keyboard/mouse)
Overlord 2 - split-screen (NOTE: 1 person may have to use keyboard/mouse)
Pid - local co-op
Sega Rally Revo - split-screen (NOTE: 1 person may have to use keyboard/mouse)
Shatter - local co-op
Sonic & SEGA All-Stars Racing - split-screen
Sonic the Hedgehog 4 - Episode II - local co-op (NOTE: Episode 1 doesn't have co-op)
Split/Second - split-screen
The Expendables 2 Videogame - local co-op

All on PC / 1 computer. Not sure about all listed games being good though :)

I would personally recommend these:
Trine
Trine 2 + Goblin Menace
Lara Croft and the Guardian of Light
Portal 2
The Cave
LEGO LotR
LotR: Return of the King
and Shatter + Beat Hazard if quick arcade is your thing.

Forgot to mention:

The Adventures of Tintin: Secret of the Unicorn - local co-op

Teeworlds

Killing floor

LotR: War in the north

get MAME, some ROMs, and you're have all the local coop you need

Half Minute Hero: Super Mega Neo Climax Ultimate Boy

I know it's a long name, but it's an interesting game especially the co-op.

Rush Brothers.

Can Team Fortress 2 be played by 2 players at the same time? I can launch 2x game if it's required.

Asterix at the olimpic games ^^

local Coop? get a boardgame or some cards

Serious sam, ricochet infinity/shatter(if you like brick breakers), and lego star wars 2-it is a must!

Geometry Wars: Retro Evolved. One person controls the ship with a gamepad, while the other fires with the mouse. It's a lot of fun.

Skylanders can be really fun.