Scott Gambler, Scott Ransom, Kona Bear, Bianchi 928 Carbon/Chorus, C'Dale Rize4
The two companies already listed (Viosport and helmetcamera.com) are probably the most used. I have the helmetcamera.com version(4 yrs now) mainly because of the housing it comes with. It can take a serious beating, personal experiance, and there are no worries if used in the rain. It is also alot easier to mount with the housing, a strip of velcro on your helmet and it is mounted. No need for a full-face helmet with either cam because you can mount them on top.
As with any helmet cam, you will need a camcorder. If you are on a tight budget go for the viosport. If not then get the helmetcam.com camera. Just my preference.
Which ever cam you get, contact the company for a remote switch for your camera. It will save you from wasted photage and increase the length of time you can use one battery