I use a bright flasher on city streets at night--front and rear. I can see fine without it, so my goal is to be seen. Is it necessary? I don't know, but I figure it doesn't hurt.

Edit: Sorry, didn't really answer the question. I use the Blackburn Flea. It's small and it isn't the brightest light ever, but it's attention getting, and I can see it lighting up street signs two blocks away. It's USB rechargeable, small, and pretty inexpensive, and I think it suits urban needs well.