I've been playing a 3D engine version of TA online for the past 2 months, it's called Spring and it's awesome.
http://taspring.clan-sy.com/
There are far better screenshots on the site than the one I've taken below, but I don't really like posting glossy full-graphics no-HUD no-name shots that look like renders. The screenshot below is from a 4vs4 game that went for about an hour.
It handles up to 5vs5 games pretty well, doesn't need too much bandwidth. It has no singleplayer, but there are several AI's for it and it can record small file-size replays easily. There's a tonne of maps, several non-TA mods made for it and being made for it. You can also spectate games, there's a fully working lobby along the lines of how the zone used to work, it's joy.
It's about as newbie-friendly as any other community on the internet, so if you don't have a clue what you're doing, don't try to learn as you go in some guy's game. People rarely have a problem with people spectating games though(unless they haven't downloaded the map). I'd never played MP TA before 2 months ago so it's pretty easy to pick up.
The download on the site is a full download for the 3D engine + TA content, you legally need a copy of TA to play Spring, but you don't need any content off the TA CD to install/play it.
Oh, the yellow line on the right is due to the fact that you can draw on the in-game map for your allies to see and also place markers down.