You already know that typing
say something into the Mac's Terminal will literally make your Mac say "something." The UsingMac blog posts a few more nifty text-to-speech commands that will make your Mac sing. Literally. Copy and paste the following to the command line:
See? Get a few more Terminal songs (one with kinda funny lyrics, even) at the UsingMac page. Then check out 10 more things you forgot your Mac can do.