I use the oval Schwinn one you can get at any mega-store. It velcros on any kind of bar end and doesn't really interfere with my grip in the drops.
Some of my the guys from the shop ride have busted me about it's size(cause they have those issues, I guess) but I can see them coming at least 1/4 mile back. And it also gives a nice wide view of their baffled faces when I leave them in the dust.
thanks for the responses. i think i may try one of the handle bar mounted ones, and the helmet mounts look like they would be a bit distracting to get used to. plus i use the same helmet for the road and trails, and taking it on and off, no matter how simple, is one less thing to contend with...
One more plug for my favorite mirror. You can quickly take in off and put it back on. I have a trick so that it is set up perfectly every time.
This mirror fastens to the helmet with a base plate and adhesive. The arm with the mirror on the end snaps into the base. The arm has ball-n-socket type pivots for adjustment. After a few rides fiddling with the adjustments, I crazy glued the sockets. I can still snap it on and off but the pivots of the arm are locked in place.
I've got one of those ones that stick into the end of your bars. Got it from Halter's and I really like it. The helmet mount / glasses mount ones are ok, but I find them distracting. Oh, and the one I have looks like an extension of the bars, not the ones that stick out at a right-angle to the bars. The mirror is kidney bean shaped.