I just finished Teenage Robot last week. It was bittersweet - I enjoyed the last episode greatly, but it's glaringly obvious that this show was cut early.
Need to check out Invader Zim; heard a lot of good things about that one.
I know that feeling...Daria, The Dead Zone, MST3K, The Goode Family. All shows I discovered after they were cancelled. It's like someone has let you in on a secret.