As a recreational cyclist in a city known for its lack of bicycle-friendliness, I never go out on the streets without a tail light designed to catch the eye of that distracted driver behind me even in the daytime. The USB bike light I'd been using was becoming ever harder to charge, so I plunked down 24 bucks or so to get a new one: a Cygolite Hotshot 100.
The "100" is because it's a 100-lumen light, but my riding partner says it's much brighter than the one it replaced. Even on a sunny day, the Hotshot is visible from up to half a mile away when set on its "daytime" blinking pattern. that's one of five different blinking patterns as well as a steady light for nighttime use. The light is designed to let you change the blink rate from leisurely to nearly frenetic.
Cygolite ships the Hotshot with both a seat post clamp and a seat stay clamp. Since, like many cyclists, I use an underseat pack, the seat post mount wasn't suitable (I note as well that it only fits round seat posts). I tried it clipped onto the loop in the underseat pack, but the light is so top-heavy it ended up pointing downward at about a 45-degree angle. So I mounted it on the seat stay. Luckily I had a length of leftover rubber from an old computer mount, otherwise the seat stay mount would have been too loose to stay in place. As it is, the clamp is closed all the way, even with the rubber shim installed.
The light charges with a USB cable (mini USB, not micro) and seems to take about 3 hours to charge. I charge mine after every use, so I don't know how long it lasts - I'm pretty sure the claimed "up to 270 hours" means only that the light can go a couple of weeks between charges if it isn't used.
Cygolite also sells a 150-lumen and a 200-lumen version, but this one's pretty much all I need for daytime use. So fat, it's a good investment.