Not sure if this has been linked before, simple but effective app for teachers.
Harmony Hollow Software - The Hat Info Page
"The Hat is a simple but handy little utility that offers a fun and easy way to automatically determine a random order from a list of any amount of names. You can even use it to pick individual names for raffle and sweepstakes winners, etc. or pick pairs of names for deciding random partnerships, complete with cool animation and sound effects"
They also have a countdown timer teachers may find useful
Cool Timer (Really Big Mode) Screenshot