here.) It has incredible potential for matching yarns with projects.
For example, I'm trying to figure out what to do with my skein of Baruffa Cashwool. It's rather thin, and I have *only* 1460 yards. There aren't that many projects in Cashwool on Ravelry yet, but the wonderful Fleegle has made a spreadsheet that compares a LOT of popular laceweight yarn. With that info, I can use Ravelry to look up other yarns of similar weights and see what other people have done with them.
Even if you don't need to compare 89 different lace yarns, check out Fleegle's blog! She's a funny, amazing lace knitter and has a lot of neat little articles about things like yarn shopping in Japan.