With the prevalence of TV on DVD, it's so much easier to own an entire series that you enjoyed watching when it aired, and then watch it over and over again.
So which series have you bought on DVD, and which do you wish you could own? Multiple answers are welcome, just post one per comment! :)
My list of what I own:
Firefly, Sliders (season 1 & 2), X-Files (1 & 2), Kindred the Embrace, and a bunch of cartoon series.