I've been impressed, so far, with the Bontrager Flare R taillight. USB charged, fairly water (fog/mist/rain) resistant, and weighs very little. They also make the Flare RT, for a bit more coin.

They've got a Helmet Mount, which works with any of the Flare or Ion headlights and taillights. With the appropriate mounting piece, the lights can be attached to the seat post, seat stays, rear of rack or bag, or the helmet.

Mounting two of them, perhaps with one on the seatpost (or rack) and one on the helmet, each set to slightly different flashing modes, you're practically "Day Glo" visible to other drivers. Bright stuff, even in the daytime.

Perhaps not the longest-lasting of all lights out there, but they're bright and can be seen at good angles alongside the bike.

Review @ Bike Light Database: click.