These look... kinda fun. http://www.bikeglowlights.com/

2 AA batteries under your saddle gets you 10 feet of lit "rope" to wrap around your bike as you see fit. $25 at REI, though it looks like there are more colors available if you go straight to the manufacturer. Anybody tried them? I'm tempted to get one for my commuter, but the funds aren't quite there for me right now.