Looking for some good PC Games!

Since school started, I'm not able to dedicate much time to gaming. Therefore, games like Starcraft 2, and League of Legends which take approx. 30 mins per game or so are not that great for me.

I'm looking more of a game that is quick, something like Minecraft or Terraria for example, where you can play for 5 mins or 50 mins, where you don't need to set time aside to play it, you can just jump in and play for as long as you want.

If that makes sense, does anyone know of any games like that? Thanks in advance :)

Depends on what kind of game you like to play though.

Guild Wars 2 - you can explore one day, do a quest the next, go along on an event if one is near the one after that, pop in for a quick pvp session, ...

Maplestory
Runescape
And you have to try Minecraft!!

Super meat boy seems very interesting i'll have to check it out even more.

I'm planning on getting Guild Wars 2 after I finish University, as it seems very addicting and could get in the way of my studying :P

Maplestory, and Runescape I played before and have lost interest in. Minecraft I'm still playing but I want to find something new before I lose my interest in that aswell :S.