SeaQuest DSV - Only had 3 seasons. Would have next season with new war against mysterious enemy faction revealed in the final episode.
Star Trek: Enterprise - Ended with 4 seasons. Turned out there was planned fifth season with NX-01 refit and Earth-Romulan War.
Sym-Bionic Titan - 20 episodes. Cartoon Network cancelled the best mecha cartoon ever made because of 'no toys connected to the cartoon' decision. Didn't finish the story.
Warehouse 13 - five seasons. My personal favorite sci-fi tv show. Had Star Trek veterans as episode guests. SyFy made poor choice on the cancellation.
Samurai Jack - 52 episodes with no end story. Should finish with a final battle between Jack and Aku.
Because of too many cancellations on good tv shows, I am no longer watch TV shows on cable anymore.