I've tried several d20 mass combat systems, to include those of The Black Company (ok) and Conan (not so ok, but here's the direct pdf link). However, I have to say I'm pretty please with the efforts of the excellent homebrew site Farland World. Their mass combat rules cover a lot of ground without being nitpicky, and there's even a link to a small program which makes it super-easy. I'm working on trying it out with Pathfinder.
So, inquiring minds want to know--what do you use for mass combats? A homebrew system? An older D&D rules supplement? One of the items I've mentioned? Do you skew towards the detailed, abstract, or somewhere in-between? Let's hear it!