AbbyMe
This site might provide a few laughs. It's pretty simple to pull off:
1. Enter the phone number of the person you want the call to appear to be from
2. Enter the number of the person you want it to call
3. Select media ('I want my baby back ribs' & Yo Ho a Pirate's Life For Me" are my personal favorites because they are so stupid.)
4. Submit.
Within seconds, person #2 will receive a phone call from person #1. When they answer, the media you selected will play, leaving person #2 to wonder why in the world person #1 just called them to play the Baby Back Ribs song. Sometimes I switch the numbers back and forth so I get to listen to the two people blame the other person for calling, then the "No you called me with that stupid f*cking song, I didn't call you with it" arguments start.
Pretty harmless (and very juvenile), but it's funny to watch develop in the office if you can keep a straight face. The other day, my coworker was in an argument with his son on the phone. Once he hung up I called from the guy's home number, so it looked like it was his son calling again. He listened to it, turned to me, and goes "First he gets me all pissed off and won't listen to what I tell him, then the little shit calls me back with that stupid "Yo ho ho pirate song." It got their spring break trip off to a rocky start. I still haven't told him it was me.