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Gruntz is a great ruleset. Well playtested and developed. I haven't had a chance to play yet, but it looks really well done.

Among fans of 15mm there's much less required connection between games and rules. Folks find a set of rules and match it to a set of miniatures they like best and there are a dizzying array of miniatures and rules.

Gruntz, Tomorrow's War, Future War Commander, Fubar (free fan set), Alien Squad leader are all sci-fi rulesets designed with 15mm as an option.

For Fantasy, Kings of War is a great choice. Though it claims to be designed for 28mm, it reads like a 15mm unit-base ruleset.

If you want to go skirmish, Song of Blades and Heroes is designed for both 28mm and 15mm and includes measurements for both. The classic HotT (Hordes of the Things) is written for 15mm. Rebel Miniatures makes the 15mm ruleset (and miniatures to go with it) Mighty Armies. Splintered Light also makes miniature sets intended for the game.

It's really hard to list all the options. Places like TMP and Lead Adventure have alot of 15mm fans that can point you in the right direction.

A few other companies to check out for 15mm minis and rules
Rebel Minis
Khurasan
15mm.co.uk (many 15mm minis and rules)
Splintered Light Miniatures
Ground Zero Games
Blue Moon Miniatures
Essex Miniatures
Peter Pig

Others here: https://sites.google.com/site/15mmfantasy/15mmfantasyranges
Best of luck in your search.