Here's The Adventure Time site. More info at Wikipedia.
MY HAT IS AWEEESOOMMMEEEEE by ~dinglehopper on deviantART
Adventure Time by ~KimchiCrusader on deviantART
What time is it? by ~Kundagi on deviantART
Adventure time buttons by *pinkfairywand on deviantART
Adventure time by ~DeidaraGS on deviantART
WHAT TIME IS IT? by ~Lysol-Jones on deviantART
adventure time by ~sumox on deviantART
This Hat...IS AWESOME by *ccWildcard on deviantART
ADVENTURE TIME by ~LEmoNmerANGuE on deviantART
And here's some animated gifs.
Finally, desktop wallpapers here and here (creator Pendleton Ward's site).