I was just reading this old review at gamespot (http://www.gamespot.com/pc/sim/allegiance/review.html) and I thought it looked extremely interesting. It is a complex team based space simulation. What made it so appealing to me was the realistic deep-space flight physics, the extensive technology tree, and the teamwork required. For example, like Natural Selection, you'll need an excellent commander to be able to win games.

This game was developed my Microsoft's research division and was way ahead of its time (as it's over 3 years old!). The reason I was looking at the review is because Microsoft just released the source code (http://research.microsoft.com/research/allegiance/ .

Some interesting quotes from the original gamespot rewview:

The strategic elements of Allegiance are every bit as complex as those in the latest popular real-time strategy games - your team needs to gather resources in order to expand its territory and improve its technology. You'll need to defend your mining ships as they harvest resources from special asteroids, and you'll need to deploy probes to detect enemy incursion. You'll have scouts to gather reconnaissance, fighters to gain space superiority, interceptors to defend your installations, and bombers to destroy the enemy's base.

Keep in mind that all of these jobs are being performed by live players, not computer AI, and you have a better idea of how complex the game is. Another interesting quote from the article:

You also have a fairly extensive technology tree that you can research, and it has several viable branching paths that will make all the difference in the long run. You'll need to become familiar with all the game's technology in order to understand how best to breach your enemies' defenses and how to counter his attacks.

Allegiance is a complicated game that's attractive for its sophistication as well as for its action, which means you'll likely find yourself playing in games with more of an upper-crust kind of crowd than you might be used to in online action games or online role-playing games.

Another aspect of this game which those of us who are used to team-based games such as BF1942 and NS might find surprising is that there can be many teams in a game, not just two. Also, flexible server rules such as a maximum number of lives (respawns) allowed per player encourages the type of players that we typically see in the TG community (i.e. NOT the bunny-hopping, teamie-killing, n00b found on most pub servers).