I had been using the previously reviewed Optical4less to purchase inexpensive prescription eyeglasses online, but I changed over to Zenni Optical two years ago. Zenni offers the same fabulous advantages of decent glasses for super cheap, but they are much faster to deliver, and have a much better selection, and their website is much easier to use and order. Reordering from the same prescription is a no-brainer, too.
Over the past two years I've ordered about 8 pairs of glasses from Zenni for different family members and myself, in all different strengths and styles, including sunglasses. The frame quality is okay (great for the price) and the optical quality is A+. My wife has extreme corrections, and I have a very odd combination of factors, while my daughter's prescription is mild. I've ordered single, bifocals and progressives - and the results have all been good. A simple correction and simple frame can cost as little as $10, but our typical glasses will cost about $35. Still a fantastic bargain. Even if you are style conscious, these are great for backup pairs.
One detail you have to pay attention when selecting a frame online is the width of the frame, which varies between models. Pay attention to the size indicator. I once ordered a pair too narrow. Delivery takes only about 2 weeks to my home in California.