F.E.A.R. probably has the best fire-team level "artificial intelligence" (AI) I have ever seen. The AI opponents improvise cover (e.g. flip over a table or move boxes), will cover each other well in fire and maneuver, and use their mix of direct fire weapons (rifles) and indirect fire weapons (grenade launchers) well.
The only game that comes close is Call of Duty 2. Your squad mates will take cover well (I've never seen them improvise cover though) and they use their weapons pretty well (as does the enemy). COD2 really has the single-envelopment down - your squad, and the enemy, do this over an over again. From a game-play perspective it gets kind of boring and predictable but it remains a solid implementation of the capability. This includes limited "vertical" envelopments - e.g. taking a balcony.
There is an SDK for F.E.A.R. and one has been announced (perhaps available) for Call of Duty 2.
I tried to use the 1.0 SDK for F.E.A.R. and found it just too hard to use - vastly harder than Unreal Editor, DOOMEdit, etc. There is a new 1.07 SDK for F.E.A.R - that the trade hype says is "much better" but that wouldn't be saying much given how hard (impossible for me) the 1.0 SDK was to use. I'd love to hear good things about the new version though given how interesting the AI is.