I've been using this bulb:
http://cgi.ebay.com/MR16-12V-21-LED-...QQcmdZViewItem
It's a "to be seen" light, not a "to see" light. I run it side-by-side with a halogen, because I feel it makes me more visible.
One thing to be careful of is that sometimes LED MR-16's are slightly bigger than halogens and have trouble fitting. This one is the same size.