Countdown

No flashy SF specific graphics, but the Countdown Calendar Dashboard widget for OS X is handy for all those important countdowns.

13 days until Revenge of the Sith!
Count the days until BSG season 2! etc etc

UPDATE
Two more specific widgets.
The Serenity countdown widget and the new Stargate episode countdown widget. Both with nice graphics, but does anyone really care that much about Stargate?!