A good starting place is
MyMovieFest. Members can upload their iMovie projects, share and rate them. There's also lots of resources too including a handful of free stuff (transitions, etc.). And from here, you can jump to some other places where you might find free stuff.
You don't need to be a member to browse the site, but you do if you want to take advantage of some of these resources. The "free" titles are a bit misleading though. They were just demos that, once I tried to use them, didn't allow me to do so.
Good luck. iMovie's cool.