I use the serfas tl-60 (shield): http://www.amazon.com/Serfas-Superbright-Usb-Taillight-Red/dp/B005M2EDG8/ref=sr_1_4?ie=UTF8&qid=1406390587&sr=8-4&keywords=serfas+rear+light

But fair warning, this light is insanely bright. Great for commuting! HORRIBLE for group rides. It does have an integrated seatpost mount with good angle adjustment though, so if you off-angle it enough it might be OK for that purpose.

(I notice the site nrhinkle lists shows the above light being the brightest tested light under $60. That does not surprise me at all :) )