Fillmore!
Super Robot Monkey Team Hyperforce Go
Medabots
Sagwa the Siamese Chinese Cat
Those are the only four I never saw a single episode of. I only actually heard of Super Robot Monkey Team Hyperforce Go, and I avoided that on purpose :P
As for the rest of them, a lot bring back great memories. Teen Titans was awesome, and I was so upset when Cartoon Network was stupid and didn't allow a contract renewal to finish up the series. Curse you Ted Turner!!!
Ah yeah, I remember Jackie Chan Adventures. That show was also awesome, but I missed a part in the middle, then the stuff got crazy fast and I was confused, so I never did finish it...Maybe I should do that sometime. Hm.
Avatar: The Last Airbender (as well as the new series The Legend of Korra) will forever hold a dear place in my heart. Both awesome shows, both with different feels with the pace they take.
Samurai Jack was AMAZING! Can't wait for the movie for it that's finally been approved. Too bad Aku's voice actor passed away, though. :/
Actually, nevermind, I could talk about all of these shows, and even with two sentences each, that'd take up so much space. I'll end here before I ramble to the character limit of the post :P