Some nice ones in this thread already!
Here are my select few, mostly just beginnings or best parts of songs:

"12KHz" (you have been warned)
Fire Fight theme (some may know it)
Happy Tree Friends theme (when my little sister calls)
TMNT theme
Malice Through the Looking Glass (Mirzadeh cover of Cradle of Filth song)
Warped - NFS3 soundtrack

I also use the Mahjong cheat sound as email/mms ringtone.