Humorous games.

[Titan Buttons]

Psychonauts was just so much squirrel burning fun.

 

[NerfedFalcon]

These gentlemen (or ladies) are all ninjas.

Serisouly, I can't think of any more to add unless no one has mentioned Portal.

They have. I also recommend Portal 2, Castle Crashers, Sonic Colours (if you have a Wii) and both Left 4 Dead games have moments.

 

[Druss the Legend]

Armed & Dangerous

Seconded.
Has to be the funniest game i ever played.

 

[Alfred Chicken]

No-one Lives Forever! Basically a slightly better Austin Powersesque spy spoof wrapped around a fps. My favourite moment is where you have to sneak around an East German town speaking to your contacts, however the person who set the code phrases has a slightly dirty mind. I'll never forget pressing an intercom and having a comedy German voice reply "Vant to come up for a game of Twister?" Also I'd like to mention Gex Enter the Gecko voiced by the great Leslie Phillips, "A little tongue now, a little tail later".

 

[DazZ.]

No One Lives Forever (2).

Stealth FPS in an over the top 60s spy style. Easily one of my favorite single player FPS games.

Edit: Bastarding ninjas, least they have good taste.

 

[Scythas]

Magicka doesn't take itself too seriously, and takes the piss out of its genre the whole way through whilst still being an immensely fun game.

 

[lady man lady]

borderlands. probably the only game that is genuinely funny in writing, not funny in glitches like oblivion (which is still a great game)

 

[Con Carne]

Armed and Dangerous for the XBOX

 

[Vibhor]

There was this game. More of an advergame. It parodied almost everything but sadly the humor couldn't make up for the gameplay. PCgamer did a feature(Saturday crapshoot) about it too.

 

[Plinglebob]

Well Conker of course

And now I have the Great Mighty Poo's song stuck in my head. (and I still find it funny that it was censored in the "Mature" platform)

Sadly, the level of humour in games seems to be dropping from what was already a very low starting point due to a mixture larger teams (meaning 1 persons philosophy and humour is less likely to show), humour being very subjective and so less popular and comedy being very hard to write.

For a modern humorous game, go for Ratchet and Clank. It has a great mix of both slapstick and wit/sarcasm as well as being a fun platformer.

 

[IamQ]

Brütal Legend was the first game to pop up in my head.

Brütal Legend. Tim Schafer's humour, Jack Black's delivery and all embedded in Heavy Metal Maythology turned up to 11.

Darr! Ninjad once again!

 

[Smertnik]

Pretty much every LucasArts adventure
Simon the Sorcerer
Psychonauts
God Hand
Bard's Tale
Viewtiful Joe
Armed & Dangerous
Overlord
Stubbs the Zombie
Strongbad and Sam&Max episodes

 

[chukrum47]

The Monkey Island games, hands down. Albeit, I only played the third one and then the more recent episodic ones, they're still the funniest games I've ever played.

God dammit I want to play Curse of Monkey Island again, but I'm not about to install Windows 98.

You could always play the remastered original two on steam instead...

-SNIP-

I tried the remastered second one a while ago, but just couldn't get into it. They need to remaster the third one! That would be sheer, concentrated awesomeness right there!
Although I might give it a try again, in my never-ending attempts to put off all my back to school work due in a fortnight...