Generally people go with either Lightman strobes or the DIY 12 volt option from All Electronics.

I used to use the All one but I went through 3 of them in 2 years - they just can't take the vibration of riding on gravel roads. They're designed to be in-place warnings, not vehicle mounted. They work fantastic when they work though, and they're cheap.

I now use a MagicShine taillight, and I'm thinking about getting another one and running two (maybe one on constant).