I previously mentioned the Doctor Who Christmas cards available on the BBC Doctor Who web page.
I am grateful to Jeff Carter for pointing out the existence of Star Wars snowflake patterns. And the official Star Wars blog has things like a gingerbread AT-AT and a mostly chocolate Ewok village. The Star Trek fan may prefer redshirt cookies.
If you prefer hard science rather than science fiction on your holiday cards, then the Hubble Space Telescope site always has a nice selection of printable cards with amazing astronomy photos on them.
Now all we need are some Higgs Boson holiday cards, and we’re all set…