There are quite a few well-known sites built using Ruby on Rails. Here is the top 10 most visited sites, according to Alexa, as of June 2008:
The list of all Rails sites is very long and include Friends for sale, iLike, 43 things, A list apart, anything from 37signals (e.g. Backpack, Basecamp), etc. If you’re interested in a comprehensive list, this is what I would call the official directory of all Rails sites out there.