The best stuff I use that\'s readily avalible (failing to get marble dust, and ceramic flakes) is this stuff that\'s used in model cars to simulate a plush interior. I bought the white brand for snow (obviously) .
I find it\'s okay in that it geives a nice, powdery snow, but when only one coat is applied, it looks like frost. So, for big snowdrifts I use a gel they sell at the hobby store for snow on larger models. It\'s messy and I don\'t like using it, but it gives big, pillowy snowdrifts I can then add a few coats of the car stuff to.
Using this as frost also has benefits. Notice how Aragorn gets frost in his beard in LOTR? This will work for that in a pinch. Plus, if you have any snow demons or such, it\'ll give them added realism.