What is one that comes to mind?
Oh man... There are a bunch... What are you looking for? Book keeping, Automatic appointment management, Support tickets and chat, Photo albums, forums, Simple blogs, Stores, Social networking like facebook or Twitter... There are hundreds that you can just drop in and go to work with. Need to start with what kind of site you would need first. I have played with a lot of them and have an idea what would be best for what. :)