There are lots of places that you can review books. The most obvious is Goodreads. Goodreads is a social media site designed around books. You can rate, review and list books. You can see what friends have read and compare your taste in books. You can start discussions about book related topics or about individual books. It integrates easily with Facebook if you want to share your latest discovery with your Facebook friends.
The other obvious place to review books is the site where you bought the book. Amazon is the biggest retailer at this point and many people review books on Amazon. But the Nook, Kobo, iBooks and other ebook retailers all have reviews as well.
If you want to take your reviewing to the next level, there are hundreds of sites that will allow you to host a blog about books. Checkout websites like WordPress or Blogger if you are interested in posting your reviews to a standalone site.