I wrote a little web app (Python and CGI) to organise a secret santa ring. You put in a load of names with email addresses, then it shuffles them, and sends an email to each person telling them to buy a present for the next person on the list (wrapping round). It meant a group of friends could do a secret santa without one person having to know who was buying for who.

There's a way to put people in groups where they shouldn't buy presents for each other, which is useful for couples of families, where they will probably be buying presents for each other anyway.