guysread.com  
"This site is made to help guys choose what they read, pick from all kinds of reading, find out what other guys like to read."

Scholastic Book Wizard

This link is to Scholastic's Book Wizard. You can search for readalikes for a certain book at a higher (or lower) reading level, or you can search by subject, genre, reading level, etc. Reading & interest levels go from 1st grade up to high school senior. Suggestions are not restricted to Scholastic books.

shmoop.com This is a very cool YA lit site.  It offers a ton of information about ya books (not all but it looks like they're growing!). It has summaries and study questions...

skokielibrary.info/s_teens/tn_books/index.asp#awards This site has award winners for teens.

Young Adult RA resources to explore 

Awards:

ala.org/teenstopten

SteamPunk

Cassandra Clare

Bettendorf Public Library's Young Adult Books in Series and Sequels - Teen series & sequels browse-able by author and series title or search-able by author, series title, or book title.

Columbus Metropolitan Library's Picture Book Index - Browse picture books by subject (they get really, really detailed).  Great for finding a list of books about "X" for that kid who seems to think he's read everything (oh yeah, and that kid's parents).  

Database of Award Winning Children's Literature - Created and maintained by fellow-California librarian Lisa Bartle, DAWCL is searchable by 13 different fields including reader's age, historical period, setting, genre, format, and more.  Resulting lists of award-winners are a great place to get young readers started.

Gnooks - My students either love this or hate it.  For recommendations you can try either Gnod's Suggestions (supply it with 3 authors you like, and it will start suggesting other similar authors) or the Map of Literature (supply it with 1 author you like, and it will provide a seizure-inducing ever-moving visual map of authors).

Mid-Continent Public Library's Juvenile Series and Sequels - Children's and teen series & sequels database with over 24,000 entries, covering birth through grade 12, and sortable by series title or subject -or- book title or author.

Monroe County Public Library's Children's Books in a Series - Children's series & sequels browse-able by author's last name.

Monster Librarian - Horror fiction recommendations for kids, teens, and adults arranged by sub-genre and series.

No Flying, No Tights (and its affiliates) - The original graphic novel review site for teens, with partner sites Sidekicks (graphic novels for kids) and The Lair (graphic novels for adults & teens).  Recommended graphic novel lists abound, by sub-genre.

Manga Suggestions

Dealing with Drama for Boys

Many of these suggestions came from LibrarianInBlack.net

Teen Books that Scared them for Life