Beyond a huge pattern resource, Ravelry has all sorts of tools to make your life better.
Projects - this is pretty basic. It allows you to keep track of all of your projects. You can track the pattern, yarn, and needles you used and you can also make notes of anything you loved or didn't love about the project.
Stash - This is also a pretty basic feature but it is a great resource. When you have your yarn stashed in Ravelry then it can be used to suggest yarns when you're looking at patterns. You can also see how to best enhance your stash. You can tell if you have more of one weight of yarn than others, or if you tend to buy lots of one colour of yarn and perhaps you are a little shy of another colour. You can also stash fibre and hand spun yarns. Really it's just nice to see all of your squishy goodness in one place!
Your pattern highlights - This is a great feature found on the right hand side of the patterns tab. Ravelry keeps track of the projects you knit, and the things you 'favourite' and suggests other patterns you may like. It is a great tool to see what new patterns are available that you might like without spending time searching.
Sales & Promo Thread - This is another great place to keep an eye out on Ravelry. It is in the Pattern forums. Often designers will offer their patterns for free or at a discount when they first come out. Many of them post in this thread. If you monitor it daily it only takes a minute to go through and you can often get a free pattern out of the deal.
What are some of your favourite features on Ravelry?