I've been far far too addicted to the orange box pack in, Portal recently, due to it's fantastically fun gameplay and absolutely wonderful ending (I won't spoil it for you, but if anyone's played it, you have to agree). I did think, god this would be fun in other source games, and low and behold, someone has obliged and given us half life 2: Portal.

It's got fairly long instructions, but I'm working through it currently because it just looks so much fun.

Check it out here.