A free set of rules, FUBAR is designed for large skirmishes. Each figure represents one person or creature, but figures are grouped into units of approximately 6 figures (although I used smaller units for my test battle and will continue to use the smaller size). Heroes and leaders may act independently. I like the scale - a large skirmish with units - over the individual focus of Song of Blades and Heroes.
The game also seems easy to modify. It was simple to turn it into a grid based game, and I was able to take the statistical details (armor mods, weapon mods, etc.) and simplify them into a few stats. The activation system is very straightforward, which is preferable for solo games (although I may modify the system to make leaders more important, akin to Dux Brit).
There is no published campaign system for FUBAR Medieval, although there is one for their Sci Fi 40K version, which appears to have some good ideas.