With Snowify you can easily turn your game into a winter wonderland.

This easy-to-use tool generates snow-meshes for your assets, automatically unwrapped and textured.
You have complete control over the snow's thickness, angle, and various other settings.

Works with Unity and Unity Pro.

More info (and video demonstration) here.
If you have any questions, comments or suggestions, please contact me by pm on the unity forum

note:
-The jeep, f16 and house in the screenshots are not included in this package, but are available for free on the unity asset store: jeep, f16, house
-It can be set up to not create snow in covered areas, however, this is an approximation, and the result will depend on the topology and detail of the source mesh.
-Does not work well with transparent objects, because the snow is generated based on the source object's mesh.