I was recently given the phone app Yarn U by the author, Mary Beth Klatt. I don't normally write product reviews, but this is worth knowing about.
Available through iTunes, Yarn U is an extensive catalogue of knitting yarns that can be sorted by fiber or name. In addition to a short description and specs such as fiber content, yardage, and cost, you'll find pros and cons for each yarn and links to free patterns. You can leave comments, share with friends, and mark favorites. There is also a feature that lets you link to manufacturers and shops and locate them through mapquest, but I couldn't get this feature to work on my phone.
At $2.99, this is a great way to learn about yarns that are currently available, with frequent updates.