Goodreads lets you mark any book you've read out of 5, mark books you would like to read, and it also makes suggestions of what you might like based on your choices. It gives links to amazon where you can buy the books too. I found it through Facebook , and it's similar in the way that you have people that you follow and you can view their reviews and lists. There are also lots of bookish quizzes to take, and lists of themed books to help you find new books.
The nice thing is that it reminded me of all the books I've read that I had kinda forgotten about. I find that my mind goes a bit blank when I go book shopping so it wold be a good tool to figure out what to buy from this site. Although since I started the book stall I've just been doing my shopping through that, when I see a book I can't bare to sell it joins the queue on my night stand.
Here is a link to my profile for any other book nerds.