I just bought one of these:
Bell Bicycle Bellistic Helmets
I had seen some guys wearing them at summer Off Road events. I love it. A lot of motorcycle helmets have elaborate venting systems that do not work, IMO. This helmet flows air! It does not have a DOT sticker, but it has a Snell rating. The helmet looks so slick that I doubt that I will ever be called on it. I purchased mine on E-Bay for $20. Just search "Downhill Helmet"