We've written a few that are available, and I've provided additional recommendations below.
Blood and Swash was originally written for pirate skirmishes, but it has been used from ancient Rome to WWII successfully. There's a little information here: link
G.A.S.L.I.G.H.T. was written initially for Victorian science fiction skirmishes and has been expanded to larger-scale skirmishes as well as pulp-genre skirmishes. See link for more information. The Web site has a quick start guide available as a free download so that you can try before you buy. These have also been used quite successfully from ancient Rome to the Philippine Insurrection as straight historical games. The original four books are now available in a single volume. The game is flexible enough that we've enjoyed games with fewer than 50 figures on the table as well as those with hundreds of figures. See my blog for some battle reports: bucksurdu.com/blog. The rules are also supported by a YahooGroup.
Both are available from Brigade Games, Old Glory, and On Military Matters.
A set of rules that I really enjoy is Brother Against Brother by Herb Gundt. I highly recommend them. While written for the American Civil War, they have additional rules for the French and Indian War. These are still my go-to set of rules for those two periods. I cannot recommend them highly enough. Well worth your money.
In addition I've heard great things about the Ganesha Games rules systems, but I haven't had a chance to try them myself. These usually start with "Songs of
" in the title. A buddy of mine says he prefers the Ganesha set of rules for the dark ages better than Saga, but I can't offer an opinion either way. I've tried some demos of Saga at conventions and had fun. I'm hoping to get into a full game at NASHCON this weekend.
You may want to check out the prolific products from Two Hour Wargames. These mostly use their unique chain reaction activation mechanism that is quite interesting. I believe that Ed has an active Yahoo Group dedicated to his rules. There is also a "starter" set of rules available for free download so that you can try before you buy.
We've recently enjoyed games with Musket and Tomahawk. The quick reference card needs some help, but the rules have produced some fun games. They've applied a pretty interesting wrinkle to the basic card activation mechanic.
There is the old standby, The Sword and the Flame. This will probably require a few more figures than your 25-75 goal, but they remain probably the most influential set of skirmish rules ever. Most of today's skirmish games (including our GASLIGHT) are in some way descended from TSAF.
Finally, the games from Too Fat Lardies have a large following and are probably worth your time to check out. I think most of their games are for slightly larger games than your 25-75 target, though.
I'm sure I missed someone's favorite and will be publicly excoriated for doing so, but these are the ones that I usually recommend to folks.
Buck Surdu