A lot of folks track combat order in D&D with index cards. I don’t know who the first person was to think of making custom index cards with a pre-printed form on them, but I first saw it at The Game Mechanics web site (great people, unfortunate choice of names).
I had just gotten back from a con where we’d run a four-party adventure with a total of five DMs, 24 players, and umpteen monsters, and the freeform index cards we used just weren’t good enough. I didn’t like the actual layout of the TGM cards, but the concept is great, and the rotate-for-character-status mechanic really improves the flow of a large combat.
My response was, of course, to come up with my own layout, adding fields and spot color to make them more useful. Along the way, I decided to increase the size from 3×5 to 4×6, greatly increasing the available space. TGM’s original cards, along with instructions on how to use them, can be found here; their forums also have several lengthy discussions on the subject.
Printing Note: Acrobat has two settings that can make it annoying to print odd-sized documents: “shrink oversized pages” and “enlarge small pages.” Turn them both off if you want the cards to come out the right size.