Well I have the answer, Rent the Runway. After signing up for free, you are told that you now have access to designer dresses at 90% off retail price AND you get to choose from 95 designers! They have a calendar application that allows you to select the date of your event and work around it with shipping time. They have a $5 insurance coverage that takes care of most accidents (spilled wine, lipstick marks) and they do all the dry cleaning! It is a pretty ingenious business idea and pretty similar to borrowing an amazing dress from your mom... if your mom is Carolina Herrera or Carrie Bradshaw. RTR has 24 occasions ranging from prom to 'meet the parents' to sorority party to rehearsal dinner and even 'bump friendly'. You are pretty much covered for any life event in need of fabulous attire.
In addition to dresses for all occasions, they also rent out accessories! If I was not clear before, this website caters to designers that most of us could never afford to have within 100 ft of our bodies; Missoni, Tibi, Badgely Mischka, etc. This may be too much for a random night out, but if life calls for something extra special, you should definitely check this website out. I should mention that this is a great resource not only for wedding guests but for brides since there are a couple of big events pre-wedding requiring fabulous outfits (rehearsal dinner for one). I have yet to give it a whirl but I've heard great things and I'll post again once I have some personal feedback! Enjoy!