JayCut is a site that has been around for awhile that I was happily reminded about from reading Free Technology 4 Teachers. JayCut is an excellent site for creating/editing videos. Users of iMovie HD will feel right at home w/ the simple user interface. Everything from effects, to videos, to cutting video (after all it's called JayCut ;-), is all here.
Finally, when a video is done being mixed together a person can upload to the web or even download it straight to their desktop.
I highly recommend giving JayCut a try by clicking here.