Sometimes it's fun to play a FPS until your eyes bleed but after a long hard day at work sometimes you just want to kick back and play something low key, something to decompress, something relaxing.

Which games (across all platforms) fit that bill for you?

There are a couple that always seem to hit the spot for me

SimCity SNES
Oblivion (just wandering around the beautiful world)
Harvest Moon/Rune Factory

How about you? Which games are the most relaxing for you?

Majesty .

Endrofun

Faerie Solitaire

Peggle, Plants vs Zombies, Bejeveled. Puzzle games in general

I'm not being a smart ass when I say this, Carmageddon.

For starters it's actually pretty hard to lose so you don't have that pressure. Second there really aren't any rules or problematic time constraints, so all of that tension is gone. Third, it's simply good sick fun, a way to blow off steam, relax, get rid of some of that pent up angst. I don't really have a job anymore but when I've had a shit day or a particularly nasty guest, a nice game of Carmageddon gets me feeling all peachy once again.

Rollercoaster Tycoon

skyrim or oblivion or other open world racing games where there are beautiful scenery to pass by

Proteus, assuming it qualifies as a 'game'.

Not GOG, but Okami on any platform. It's just so darn... Life-affirming.

As strange as it sounds, I found it relaxing at times to play a modern GTA game and just ride around the place listening to the radio.

X3: Terran Conflict.... floating in space is epic
Flower - but that's a console only i think
Audiosurf - use your music float around have fun

I can FPS in my sleep drunk with one eye and missing my index finger. That said i find these relaxing for me. The reasoning behind some of the these is i have the entire game memorized, every spawn location, hit boxes, paths to take.. so i can just float though a level without thinking at all.... mindless fun! Others with Dynamic content I just have played it enough to know the core mechanics so its no longer challenging just, relaxing....
Painkiller
Serious Sam
Unreal
Quake 2
Borderlands

I also love driving around:
Midnight Club
Grand Theft Auto 4
GTA5 has some of the best mini games I've ever played!
Saints Row 2-4

NO2 on the PSone.... ultimate relaxation for me!
Spider another PSone game... best soundtrack ever in the history of gaming!

This maybe seems strange but first three Harry Potter games were maybe the most relaxing games I ever played, and I'm not even a HP fan (nor hater). Flipendo!

+1 to that one. The first Borderlands was so over-the-top grim and dusty and ultraviolent that it sort of goes all the way past shocking and wraps back around to zen. The music is really ambient and relaxing too. I could lose hours playing that and feel almost like I'd taken a nap. Doubly so when playing with my girlfriend, because then we'd also get the maniacal giggles every so often.