Ruby on Rails is already going strong on 43things. Learn Ruby is #2 and Write an application on Rails is #8 according to the 43things Zeitgeist. So if you already have done or are doing these things, please do let 43things know and share your experience with all the other people currently working on it.
About the experience of building 43things itself with Ruby on Rails, Erik Benson writes:
My first exposure to Ruby on Rails was when I sat down and started building the prototype for this site (43 Things). From the start, it was easy to learn, full of wonderous surprises, and it encouraged excellent coding practices such as separation of controllers, models, and views, unit testing, and documentation.
It has only been a little over 3 months since I started working in Rails, and 43 Things will be launching soon having been written completely with it. Having used several other languages and frameworks to build websites in the past for both personal and professional sites (I used to work at Amazon), I can say that this has been the most enjoyable one yet.