Third party publishers and what they were doing with the game are what brought me back to D&D around 2003 so I've a LOT of it. I have gotten considerable joy and mileage out of the Malhavoc Press products - When the Sky Falls, Requiem for a God, the Anger of Angels, as well as Beyond Countless Doorways, and Chaositech.
Ancient Kingdoms Mesopotamia (Swords & Sorcery) - I could go further with the Swords & Sorcery stuff, but that's primarily adventures (especially the earlier ones) but those aren't specifically "rules supplements." Add in almost all of the Dreamscarred Press psionics stuff.
I'm sure there's more but those are the highlights.
I would really like to say Keith Baker's the Complete Guide to Wererats (by Goodman Games) but on looking I don't think it's open content. Pity that.