This is probably an old one, but I've just discovered and think it's pretty cool.
Open up a terminal on your Mac and copy and paste the following into the terminal and press enter:
osascript -e 'say "oh This is a silly song silly song silly song this is the silliest song ive ever ever heard So why keep you listening listening listening while you are supposed to work to work to work to work its because i hate my job hate my job hate my job its because i hate my job more than anything else No its because youve no life youve no life youve no life and you better go get one after forwarding this crap" using "cellos"'
For those who don't have a Mac, I've recorded it to MP3 for you.
You Mac will sing to you. From my own little investigating, it seems to only be able to sing in two different "tunes", the one above and the other to Pomp & Circumstance March No. 1 (type the following into your terminal:
say -v Good ooo ooo ooo ooo ooo ooo ooo ooo ooo ooo ooo ) but you can change the words to suit your needs.