If you haven't looked recently, Microsoft does occasionally update its Windows 7 Themes page with new themepacks, providing your PC desktop with high quality background photos, color and sound schemes, and more. And sure enough, there are some nice new ones there now (I particularly like the Bing ones), including some fun themes for the holidays:

Eerie Autumn

Harvest Time

Holiday Lights

There are some other fun new themes in there, too, including one for Halo: Reach.

Thanks to Marlon H. for the link.