Pacman Champion Edition DX. That's about as chill as I get.

n2o Nitrous Oxide there exists no better game to chill out too than this....

Osmos
Sim City 4
Risk II
Some puzzles like Droplitz, Chime, etc.

Heroes of Might and Magic 3. I sometimes leave it on for days, coming back to it when I'm in the mood or need a breather from other things. It's light, bright, upbeat, and fun without being too intense or demanding much concentration, so I'm comfortable playing it in almost any mood or any state of alertness or sobriety.

I also like Moraff's Mahjong, but since I race against the clock in that game, it can make me concentrate enough to get burnt out, or get too wired to sleep. Still, it's a great game to play when waking up and trying to get your brain alert, or when coming home from work and trying to flush all the memories of the day out of your system and get back to a good or neutral mood. It takes me right out of my world, 4 or 5 minutes at a time.

Osmos was awesome.

Lately for me the chill out game has been Puzzle Quest. I mean you can play it for 5 minutes of 5 hours and it's not the bit stressful.

Ooo, I love this topic! I've been trying to come up with a good list of games like this. Here are my favorites:

Windosill - beautiful, surreal, and slow-paced. Perfect. One of my favorite games.
Flower - See above, although the later levels are disappointing.
Norrland - Sensing a pattern yet?
The Katamari games - The only games of any length on my list, unfortunately. Also my PS2 is busted so I need to get a new one or the PS3 game.

I've had others recommended to me that I either haven't played yet or didn't quite work for me:
Noctis IV - a bit too convoluted, but a great game
Rez - fantastic but a bit too intense for chillin'
Aquaria - too shmuppy
Knytt Stories - occasionally difficult
Flow - haven't played
The Void - great atmosphere, but I hate the mouse gesture crap

There's also The Path, if you like really really boring art games with great atmosphere.