Read or Die, or Blue Submarine 6.
Hell a lot of anime is meant to made into a whole series, but turns into OVA's because of budget contraints. *pout*
Another vote would be for Greg the bunny. I have all those on DVD, even though i had to make it myself...
Clerks was pretty good too.. Though I can understand ABC's point; It does cost a lot to make a cartoon, and if no one is watching that's money that can be spent on other projects.