this is one I found this year, and I like it because it uses plain old worsted-weight yarn, which I have, and because it's actually pretty simple: no picots or anything fancy, just six points. Each snowflake is meant to be one motif in a garland, but just one makes a good tree ornament.
Are you looking for last-minute things to crochet? Check out Crochet.About.com ; they keep adding "best of" posts with free patterns. There are links to the best ornaments and so on on All Free Crochet, on Craftsy, on Ravelry, and so on.