robobrien: I want to try the fantasy version of TA that Cavedog made, even though many of the old reviews generally sounded disappointed with it.
Lexor: Total Annihilation: Kingdoms? I played it. In fact I was the only person from my friends who finished all scenarios in single player campaign - and there was much more of them than in any other game. I've just found by googling there was 48 scenarios, pretty big number, and only a few starting ones were short and simple. I must say I liked it much more than original TA but that maybe was because of fact that I prefer fantasy settings over sci-fi settings. From what I remember, I enjoyed it but, unfortunately, I've never played it online.
I own a copy of TA: Kingdoms I can confirm that it is mediocre. However, they never skimp on their games, there's a ton of levels iirc.
orcishgamer: I know this is solved, but you can mod the hell out of TA, making its replayability very high.
Drelmanes: Can you, or someone else, recommend me good mods?
Honestly there's so much, from unit additions to full on game mechanics changes, you should really take a peek and see what you're interested in yourself.
I recall this being sort of the official hangout back in the day:
http://www.tauniverse.com/
but this one comes up on Google too (seems to have a good writeup of the kinds of stuff out there too):
http://www.rakrent.com/rtsc/html/ta4.htm I'd check those two sites, do some googling to make sure they're the real deal and don't shovel viruses and try some stuff out. See if you can get a friend to play with you, that's what made this game fun.