The YALSA part of ALA is good, but let me also recommend the book review section of the School Library Journal and one I stumbled on - San Jose Public and San Jose State University combined site contains a 'Teen Reading Lists' section.
A good all around recommendation site is What Should I Read Next. You submit a book you enjoyed reading and it gives you a list of books you may also like. I knew it was good when I entered Sabriel by Garth Nix and one of the books it listed was Pope Joan by Donna Cross, which is another one of my favorite books, but a completely different kind of book compared to Sabriel.
LibraryThing also has a good (but rather hidden) recommendation tool called BookSuggester. There is also the Unsuggester to see what books you probably shouldn't waste your time reading. If you are looking for book recommendations for yourself, you should set up a LT account and enter your favorite books; LT will then suggest books based on your personal library. You can also then see how likely you are to enjoy specific books, by using the "Will You Like This" gadget found on each book profile.
With Valentine's Day coming up, I keep getting 'love your library' emails. One of them led me to ilovelibraries.org which has a book recommendation section here. It includes a section for adults, young adults, and children.
I love the What Should I Read Next. I went around to the English teacher's in my school and gave them the website address. They were really excited. We are going to add this to our Media Center homepage.
Thanks
Thing 35 has some book recommendation sites, including What Should I read Next. It would be interesting to hear if HS students like any of those. ReadingTrails looks interesting.
@Ann I think that Reading Trails has an interesting concept and I'm hoping the website continues to grow; it is obviously still in the early stages of development. Then again, so was What Should I Read Next, when I first came across it.
Type in the author of a book you liked and get suggestions of other authors you might like too. I think that a lot of my teen readers will enjoy this. I know it worked really well when I put in some authors I enjoy reading