Holiday Lemmings is a Christmas-themed take on the Psygnosis classic. I'm almost positive it was shareware.

In all Munchers games, the reserve munchers will wear Santa hats from December 1-December 25.

I'm pretty sure one of the After Dark modules draws a Christmas tree if you set your clock to December 25. I think it's the fractal forest one, but I can't remember offhand. I know it's not one in the original After Dark set and was released with some other package or book.