This script creates small rectangular selections and auto-white-balances them. Copy it into the scripts folder from GIMP, you will find it then under Filters -> Light and Shadow -> White Balance Puzzle.
Be careful with the feather option, it consumes a lot of cpu power. This script gives a strange result, so it just fits in rare conditions:
I’m using it for image analysis. It “detects” image edits in low contrast areas.