dont forget after the 7 seasons, there is an hour special "Say Goodnight to the Bad Guys" that links the end of season 7 to their 2nd movie "Countdown to Liquor Day." Their first movie doesn't really tie in to the series, but it's hilarious.
Also there is a black and white pilot movie (I think about an hour long) that takes place before the actual series pilot episode. It's a little darker, and a bit different with the settings but worth watching.
EDIT: here it is
Just to further cement my promotion of this show, my old housemate wanted to watch the series so in 3 days we sat and watched every single episode and movie
I can't say that about any other show