If you've really wanted to try the WWI FPS Tannenberg you're in luck, it's free to download and play for a few days plus it's on sale along side other games from M2H.
Quite a fan of Tannenberg myself, it has some fantastic sound design and the open levels are great. Difficult though, as it's somewhat focused on realism so you can't just run across the map expecting to be able to dodge bullets.
Tannenberg Feature Highlight:
- Authentic WW1 atmosphere; accuracy in everything from maps and weaponry to uniforms
- Tactical squad-based FPS; play as Russian, Roumanian, Latvian, Austro-Hungarian, German and Bulgarian troops
- Wide-ranging landscapes; large maps each with weather variants from snow to summer sun
- 64 player Maneuver game mode; capture strategic sectors in grand battles of movement
- Never fight alone; squad gameplay and AI bots for Maneuver so you can join epic battles at any time
Sounds like another big update is coming soon to Tannenberg too, as they've recently been teasing a new map for it.
Other titles from M2H are also on sale including the party game Marooners, the other WWI FPS Verdun and their stunt game Crash Drive 2 which includes a free copy to gift to a friend when you purchase.