I am currently using an Oregon Scientifics helmet cam. It is cheap, around $75 and works pretty well, but I am sure that there are better options.
I keep it mounted on the front rack of my ride and it can capture about 1 hour of video on a 1 GB SD card. This works well and I just carry several cards on long days. It is waterproof as well, which is huge.
The microphone is not so good, very affected by wind and engine noise, so it is unusable at this point, thus why I put music in all of my videos. I am still working on this as I would like to hear engine sounds on my videos as well.
My next step is to put another cam on the back rack and capture both perspectives at the same time.
Oh well. Thanks for checking out my videos. There are alot on my Youtube, snow, mud, hills, streams, etc.