Hey,
There are two WWI Flight Sims out there that I can think of offhand:
The first is Over Flanders Fields, which is (basically) an outrageously modded version of Microsoft's Combat Flight Simulator 3.
The second is Rise of Flight. This is a relatively new game which originates in Russia.
Both are only available over the net (just google them) as far as I know.
They are also pretty extreme, ultra-realistic simulation games, which makes them both a little out of my league. I was a huge RB fan; these are much harder to learn and play. Frankly, for my taste, life's just too short to spend hours (and I mean HOURS) trying to learn how to take off and land a Fokker D-VII. With that said, they are both gorgeous. OFF is a serious labor of love for some hardcore flight sim guys, and RoF is just as good looking. One thing to note - OFF has accumulated bunches of flyable aircraft over the years. RoF has a much limited set of AC's and they sell them as downloadable content. The original game had (IIRC) four aircraft; they have come out with additional ones, but each costs you.
Hope this helps,
Gunner