http://www.bgmod.com/

This is a mod for Half-Life and it is absolutely fantastic. You just need to get over the fact this is the second beta and it is very unfinished.

The models are poor and the animation needs work and knives/bayonets/swords are a bugger to use, but I love it and so do a lot of people

It's set in the American War of Independance and Pits the british and Loyalists against the Colonalialist troops. You have muskets that take about 30 seconds to load and are inherrantly innacurate but with practice you soon learn the knack of picking off targets with them.

If you haven't tried it download it and give it a go, I play mostly on the Multiplay public servers