Schoolhouse Rock!, on ABC
This interstitial series originally ran from 1973-85; there were a couple of later runs, as well. I'm not even sure whether I was aware of the series's existence until the album School House Rock! Rocks was released in 1996 (when I was in college). But I'd definitely like to watch the whole series someday.

Super Milk Chan, on Fuji TV (Japan)
This started as a microseries in 1998, before becoming a regular series in 2000. I'm sure I saw both versions on Adult Swim in 2004, but now I don't remember the microseries version at all, so I'd like to rewatch it someday, if possible.