I adore Ravelry for many reasons, not least of which is the insane levels of organization it encourages. Quite a while ago, I photographed and cataloged my entire yarn stash, and I've done a pretty good job of keeping it updated. If you link to yarn that's in your stash when you create a project, then Ravelry will keep track of how much of that yarn you have left based on your project's yardage (if you enter it), and link to any projects that used that yarn on the yarn's page. I love looking at the page for one of my skeins and seeing all the projects I've made with it, especially my lovely acrylic that I used for multiple amigurumi (and still haven't used up, ugh).
One thing that bothered me, though, is that I didn't have an accurate way of measuring how much yarn I had used in my projects, and how much yarn I had left in my stash. So my stash page on Ravelry was good for keeping track of the yarn I own, but it wasn't actually a good representation of how much I really had.
Then, I got a kitchen scale for Christmas!
Jen of the Commuter Knitter podcast mentioned recently that she has been weighing her projects and using that to keep her Ravelry stash up-to-date, so I was inspired to spend my Saturday afternoon doing the same thing. I tracked down all of my projects that I haven't given away, starting way back with a crocheted bag from 2008, and weighed them. Then, based on Ravelry's data about how many grams are in a skein of the yarn I had used, I calculated how many skeins my project had taken. Now, many of my project pages have accurate yardage on them! Yayyy! And, I know almost exactly how much yarn I have left over from those projects in my stash! Yayyy!
Many of my acrylic skeins unfortunately can't benefit from the same automagic updating, because I used them for multiple amigurumi. I can't weigh those, because they have been given away, they use yarn from multiple skeins, and/or they're stuffed so I can't weigh the yarn alone. So, I just weighed the remnants that are left in my stash, and calculated how much of a skein I have of those. I put that amount into Ravelry as my "amount stashed," which isn't exactly how it's supposed to work - ideally, you start with the number of whole skeins you originally had, keep track of how much you use in your projects, and Ravelry calculates your current amount stashed from there. The way I'm doing it for my acrylic partial skeins, I'll have to manually update it if I use some of that yarn. Not a big deal, though.
Now that I have a pretty exact idea of how many yards I have of all my yarn, I can do some advanced searches on Ravelry for projects that will use up x amount of y weight yarn! Even though working from my stash doesn't inspire near as much excitement as buying new yarn, I had so much fun last night looking for patterns I can use. I'm linking to possibilities in the notes section of each yarn's page, so I'll remember them when I decide it's time to use some stash. I love Ravelry.