Death Note - anime
K-ON! and K-ON!! - anime, if you like moe
Elfen Lied, only if you like bloody anime. Both anime and manga
The Melancholy of Haruhi Suzumiya - anime and/or visual novel
Steins Gate and Chaos Head if you like really vague anime's with all sorts of difficult theories.
Code Geass
I'll post more when I get back home ^^