every monday ( apparantly )

starting with the Codematsers bundle
https://www.humblebundle.com/flash

24 hours only.

PWYW:
Overlord (Steam)
Overlord: Raising Hell DLC (Steam)
Operation Flashpoint: Red River (Steam)
Operation Flashpoint: Dragon Rising (Steam)
Rise of the Argonauts (Steam)

$6 or more:
DiRT Showdown (Steam)
DiRT 3 (Steam)
Overlord II (Steam)

orginal thread here http://www.gog.com/forum/general/humble_flash_rebundle