I've been going with EyeBuyDirect the last few times I've gotten new glasses. I've used Zenni a few times, and they're definitely on the list of places I check, but EBD has a better selection. I save so much money on frames I can afford better lenses, and they run 2 for 1 deals pretty often. So I get a pair of regulars and a pair of polarized sunglasses.
You can upload a picture and then try on the glasses virtually and judge that way.