iMovie has several preset export settings. you can generate web-useable files that way.
that said, i use quicktime pro for granularity in exports, and for its applescriptable features. well worth 30 bucks (or the grand i paid for final cut pro).
try iMovie's offerings. if they don't suit, upgrade to qt pro. you'll also get the browser plug-in enhancements with it. i find it a very versatile tool.