Are you looking for new bikes or used ones?

Or both?

New, I always recommend the Kona Smoke ($350) or Kona Dew ($400). I'm pretty boring and predictable that way. (Smoke is an awesome commuter. Just saying...)
Used, anything that feels good and meets your budget! Girly-looking bikes are less likely to be stolen, but that's no guarantee of safety.

If you have a Bike Kitchen or Bike Works or other bike-assistance program near you that sells refurbished bikes to fund their program, I'd definitely look into that! http://forums.teamestrogen.com/showthread.php?t=24656