Themes
The Free Themes Directory at WordPress.org just updated its list of featured themes, and now includes two different Christmas-related themes:
Plug-ins
The most commonly used holiday/Winter plug-in is the snow plug-in featured on WordPress.com:
There are also 35 different plug-ins in the repository that match a search for “Christmas.” I tried looking for anything freely available for other holidays, but I have yet to find anything … so if you want to write a plug-in to celebrate Hanukkah, I think it would be very welcome! 🙂