Good "In-between-homework" games?

Super House of Dead Ninjas. Great rogue-like platform game.

Mountain

Maybe study and play after it?

Osmos, Eufloria.

Type:Rider, Lyne, and Faster Than Light.

Audiosurf
Beat Hazard
Sir, You Are Being Hunted
Nidhogg (Recommended)
Surgeon Simulator

Roguemek if you got work to do but need a mental break.

XCOM enemy unknown/within. fallen enchantress is pretty good too but requires /some/ dedication.

Depth is pretty good, it's on Steam

Zuma's Revenge

Tropico Series, I used to play one campaign mission every time I take a break.Then I finished the 3rd and 4th games and got a 3,29 GPA that year.Not as time consuming as Civ V and pretty playable in study breaks.

Antichamber, +5 to INT ... +5 to INT ... and so on.Makes your braincells work and gives you some good satisfaction after you solve a hard puzzle.Recommended if you need some ... +INT !

Mount & Blade Warband with MODS ! Play until you lose an important battle then RAGEQUIT, keep studying for an hour. REPEAT

Super Meat Boy? You'll be happy when you can go back to your homework :D
Honestly, I would recommend any hidden object game as well. They are quite slow paced, have an intresting story (at least the better ones, there is quite a few of them), keeps you thinking and your eye and wits sharp without overtaxing it. I think it's a good choice if you don't want to completely stop between learning sessions, just slow down a little. :)
I also recommend you Papo & Yo. It's an exploration-based platformer game with a great story. There are quite a lot saving points, so easy to stop, then catch up later - and the game is beautiful. You will still need to think about what to do and how to do, but the experience was so refreshing for me :)
Or if you'd want games that don't have a story, so easier to start and stop:
Circuits
Auditorium

6180 The Moon is pretty cool. It has an interesting screen wrap mechanic as well.

Super Amazing Wagon Adventure, really worth it!

No votes for Counter-strike?

i play racing simulators, a couple of laps around a track clears my head and relaxes me.

Asseto Corsa

Raceroom Racing Experience(Free to Play)

SimRaceWay(Free to play and even you can get payed for playing lol, is really hard tough)

I usually turn to rogue-likes or hidden object games for this.

For roguelikes, I'm absolutely in love with WazHack. After it reminds me how incompetent I am at life, I can go back to my homework.
As for hidden object games, I really like the ones from Artifex Mundi, especially the Nightmares From The Deep games.

I really wish I had more suggestions for you, but most other genres suck me in for way too long. Sometimes horror games can do it for me -- they have a tendency to clear my head, and there's a couple that I just can't play for too long in one sitting, like Outlast. And sometimes multiplayer games, but it's been quite awhile since I touched Insurgency. =p

FTL: Faster than Light, and binding of Isaac you can rest after you clear a room, Five Nights at Freddy's, Long live the Queen, and pretty much any simulator or MMO like pokemonMMO or something like that

also 100% orange juice

Pokemon TCGO

Hmm, Let's see...

Binding of Isaac (Rebirth is being released today), Risk of Rain, MxPixel (great to cleanse your brain), FTL: Faster than light, Hearthstone, Beat Hazard (similar reasons to McPixel).

Nimble Quest - it's simple, but surprisingly addictive, and personally I find playing it really calming, even though it's not what I would call an "ambient game".

http://store.steampowered.com/app/259780/

You might want to wait for a sale though, as 4.99 euros might be too much for what it offers. Myself, I got it for 0.49 and, as somebody said in a review, "best spent 50 cents ever". :D