So, for all of you out there who are not yet on ravelry, you should join! It's a great resource for yarn crafters because it was specifically designed for knitters and crocheters. Oh, and it's free! There are so many things you can do on it, but here's what I've used it for.
The notebook is awesome; it includes a message box for communicating with other ravelers, a library for keeping track of what pattern books you own as well as digital patterns, a 'stash' for keeping track of the types, colors, and quantities of yarn you own, a project list for ongoing and completed projects, and a queue for lining up future projects (or just saving the patterns for future reference). There are several other aspects to this tab, but that's what I primarily use it for. You can, for instance have friends on ravelry and you can navigate to their notebooks through this.
Ravelry does more than just keep track of what you've done and how you've done it (which is incredibly valuable). There is a patterns tab where you can search for patterns and narrow your search based on type of craft (knit or crochet), difficulty, price (free or otherwise), category (hat, afghan, dishcloth, etc), popularity, or newness of the pattern. I've found some great patterns to try on ravelry, and all of my patterns are listed there. They also have groups and forums tabs where you can find local yarn crafters and get help with patterns as needed.
Ravelry was especially valuable to me on the Celtic Star Blanket because the gray color of yarn I was using was discontinued and I needed one more skein to finish it. I went on ravelry and searched for that yarn in other people's stashes (actually pretty easy to do), and sent them a message offering to buy it from them. This worked, and I now have a complete afghan. This wouldn't have happened if I didn't have ravelry.
More features are available but those are the ones that I have found particularly useful. Ravelry can be a fun way to spend some time and get some inspiration. It's also a great way to keep track of things you've made, how you made them, and to keep track of your inventory.
If you want to look me up I'm hazel92 on there. You can check out my projects. I don't blog about all of them, so it won't all be redundant.