I think there may have been a thread about WW1 aircraft a while back which listed a lot of what's available. Here's another list
Payware
Airco DH2 (Aeroplane Heaven)
Bristol M1C Bullet (Chris Herring)
De Havilland DH5 (Chris Herring)
Sopwith 1½ Strutter (Chris Herring)
Fokker DVIII/EV (Blutwerks)
Fokker EIII/EIV (Aeroplane Heaven)
SE5a (Alphasim)
WW1 Fighters pack (First Class Simulations) - Fokker Dr.1, Sopwith Pup, Nieuport 17, Avro 504 & Zeppelin LZ62
Freeware
Airco DH2 (Chris Herring)
Albatros D III (Stuart Green)
Avro 504K (Stephen Barstow (Stiz))
Bristol Scout C/D (Stephen O'Leary)
Bristol Fighter F2B (Robert Bruce)
Bristol Fighter F2B (Frank Elton)
Caudron G4 (Stuart Green)
De Havilland DH9 (Chris Herring)
De Havilland DH4 (Chris Herring)
Felixstowe F2A (Stuart Green)
Fokker DR1 (Frank Elton)
Fokker DVIII (Frank Elton)
Fokker DVII (Stuart Green)
Fokker DR1 (Simtech)
Handley Page O/400 (Simtech)
Pfalz DIIIa (Stuart Green)
Pfalz DR1 (Frank Elton)
SE5a (Stuart Green)
Sopwith Pup (Stephen O'Leary)
Sopwith Cuckoo (Stuart Green)
Sopwith Snipe (Stuart Green)
Sopwith Camel (Dave Eckert)
Sopwith Triplane (Frank Elton)
Spad VII (Stuart Green)
Spad XIII (Simtech)
Zeppelin Staaken RVI (Stuart Green)