Thought I'd start a thread so people could mention some great series they've seen. I'm always up for something new.
Firefly was one of my favorite series ever. It was created by Joss Whedon, and origionally aired on FOX, which was the reason it failed. It was cut short before the first season even finished. The main reason why people didn't get into this series when it was on tv was because FOX decided to air the episodes out of order. They aired episode 2, 3, 6, 7, 8, 4, 5, 9, 10, 14, then 1, with episodes 11, 12, and 13 never airing. Way to go FOX. ..Anyways, if you haven't seen this series, you should do so. It's amazing.
Band of Brothers was another great series, from HBO. It was written and directed by a few different people, such as Steven Spielberg, and Tom Hanks. It takes place during WWII, shortly after the attack on Pearl Harbor. ..I'm not a huge fan of war movies, but this is by far the best war "movie" (series) I've ever seen.
The writers/directors are actually making a new series now, called The Pacific War. Unfortunately, it's not sceduled to come out until June 2007-2008.

So, in the meantime, go check out Band of Brothers, if you haven't already.
Add to the list, I know I will.