Our school uses two leveling systems - (1) Reading Recovery Levels and (2) Fountas and Pinnell Guided Reading Levels. The systems are aligned and books are also leveled using color dot stickers. You'll find a sheet in the resources below that shows the correlations between the colored dots and the two leveling systems. In 2007 I started to level my classroom library. This was after I began adding my books to my Library Thing database, so needless to say, my database online won't have the book levels. I wish it did, but it would be REALLY time consuming to go back and add the leveling information to each book.
When leveling a book, I use several sites to determine a book's level. I usually try them in order by the number they are listed until I find the level. If I can't find the level, I use my best judgment and knowledge of other books that are similar (series, authors, etc.).
Reading Level Comps Chart from Harcourt: http://www.oema.net/lexiles/ReadingLevelComps.pdf