I just found a really nice open-source effort - www.americasarmy.com. This is a website of the US Army - they developed a first-person game based on the Unreal engine that includes many very realistic missions and physics models, for use in training and introducing the public to the Army. Great concept!

I have downloaded it and hope to install it soon. Be warned, its a 260mb download. You can also order it on CD with bonus missions for about $5 or $6.