These are a set of rules for gladiatorial combat in the arenas of Rome, adapated and expanded from a set by Gary Comardo. The current version is v1.4 (October 2005):
Steve Barber Models - A comprehensive range of nice-looking figures at the smaller end of the scale.
Gladiator Games - A full range of types, with an emphasis on the more common heavy and medium gladiators and retiarii. Not the best looking figures in the world, but they paint up OK. This range forms the core of my own collection and is recommended as a starting point for building up a collection.
Foundry - Very nice figures indeed, let down by peculiar marketing which means that they may or may not be available to buy at any given time, and are packaged in sets which mix useful figures with much less useful ones. The range includes all of the common types, as well as female gladiators and dwarves.
West Wind - An extensive range of types in sensible pack groupings. More 'cartoony' than other ranges, with some odd poses and equipment that seems to be drawn as much from fantasy as history.
Irregular Miniatures - A smallish range of basic types, some with unusual, but not unconvincing armour.
Castaway Arts - An Australian company who do a small range of gladiators. More slender than other 25/28mm offerings, they still mix well with other manufacturers and are worth getting to add variety to your collection. Seperate weapons and shields allow plenty of choice as to how they can be classified.
Bronze Age Miniatures - At present they only do a pair of female gladiators, both of which look very nice, if a little fanciful. They look to me to be at the upper end of the 28mm scale, though.
Shadowforge - More female gladiators than you can shake a stick at. In fact they don't do any male ones at all. The Valkyrie beserkers would make interesting barbarian gladiators as well.
15mm:
Museum Miniatures - A good selection of all of the classic types, plus a few unusual ones (including a pugiles). A good range with which to start off your collection.
Outpost - A small selection of mainly sword-armed heavy and medium types, plus the obligatory retiarius.
M.Y. Miniatures - Some, if not all, of the figures come with separate weapons, allowing a certain amount of variety in your fighters.
Miniature Wars - An italian company that does two sets each of six figures covering all of the basic types. Possibly the best looking 15mm gladiators I have seen. (Click on '15mm', and then 'Civili').