Minecraft, if you got some friends and set up a server it´s a blast. Of course it´s almost as fun single player aswell.
Edit: Magicka is good aswell.
Magicka is by far the best in that price range. Why are people saying Minecraft? It is utterly pointless, boring and oh so wonderfully visual (not). I played it for several weeks just to try and understand why so many people are into it, and just can not get past the fact that it is terrible ugh, morons.
Amnesia was definitely fun. Throwing my vote behind minecraft as well. The gameplay is extremely addicting. It's easy to lose hours and hours without realizing it :P
Perhaps you can find GTA SA cheap somewhere since it's a bit old
Buy max payne bundle from steam.
No under £10, but you can get the Complete Collection of Guild Wars (all the 4 games) for like £17. It's an MMORPG/CORPG without subscription fees and definitely worth the price.