Adobe Premiere Elements 9, right now. The free trial puts a watermark on final videos, but you can get a really good feel for the app. I was using Premiere Pro, but the older version I had couldn't handle the HD video. Premiere Elements a little too simplistic for my tastes, coming from Premiere Pro, but Elements does an okay job for now.

And doesn't Mac come with an app that natively handles HD H.264 MP4 video? If you're running Windows, you can convert the MP4 to WMV and use Windows Movie Maker to edit the videos.