War and Empire are sold by Forged in Battle: link
Essex: Macedonian and Punic Wars, Romans, Gallic
Gladiator Miniatures: link
QRF: link
Irregular Miniatures: link
Battle Honors (no photos): link
Also sold by Old Glory 15s. These are really nice (I think by Tony Barton?), but really big. Not compatible with the above, nor with Old Glory 15s Romans or Carthaginians.
Xyston: link
Also really big, and only for really early eras (1st Punic War).
Old Glory 25s sells the Blue Moon 18mm range, which includes Gauls and late Republican Romans. These are really nice, but really big. The Romans can probably be used for Principes, cavalry and officers, and for Triarii if you add spears to the right poses, but there are no suitable Hastati or Velites figures.
I'm sure I'm forgetting a few…
- Ix