Update (2020): I removed the link because a lot has changed in the last 13 years. If you really want this terrible, probably broken code, send me a note.
I wrote an RSVP program for our wedding in Ruby on Rails. It includes live search and editing using AJAX which was ridiculously easy with Rails. It also has a shadow table to which all changes are written in case one of your guests accidentally deletes everything somehow. Here is the source code, including the pretty graphics. Feel free to use it for anything and let me know if you find it useful.