The first time I used Ravelry's advanced search, I was floored. You can search by type of project, knit/crochet, free/not free/available for download/in a book I own, intended gender/age/size of wearer, size of yarn, amount of yarn, techniques and construction features, language the pattern's written in, and some other categories I'm forgetting. And you can search by any combination of categories. I'm sure it took a tremendous amount of time to code, and I know that it took an army of volunteers to tag every pattern in the database for it, but it's remarkable.