This page lists flag challenges of AH.com other than the Weekly Flag Challenge. These are mostly FCs from AH.com's past, prior to the establishment of the WFC in 2009.
AH.com Official Flag Contest Discussion
AH.com Official Flag Contest Round 1
AH.com Official Flag Contest Round 2
AH.com Official Flag Contest Round 3
AH.com Official Flag Contest The Final Flag Contest
Flag Mission - Taking two completely different flags, the flag of Scotland and the flag of Manchuria (Manchukuo) and combining them into a single Flag that incorporates the symbolism of both the flags. The thread also includes the challenge of combining Bolivia and Toronto in the same way.
What if all countries had to use Nordic Cross style flags ? - A rather loose and informal competition based on the premise in the title. Done more for fun than anything else.