Handbrake is great!! It’s free and does a great job of compressing video.

For web movies choose Fast 720p30 (Modified) as your preset and then also select the web optimized check box. This can reduce the file size on exported iMovies by close to 75%.  

I always save the new movie as <oldMovieName>Web.mp4 so that I can easily find the compressed and uncompressed versions and check their quality against each other.

Another great feature about Handbrake is that it will queue things up for you.