I think that there is a misconception that great cleave doesn't proc the off-hand attacks when used on a monk. In any case, I know I was under this impression. Fortunately, that impression is wrong.... some of the time.
While it is true that Cleave does not ever proc off-hand attacks, great cleave does indeed work as you would expect for some races and sexes. My character where I initially noticed this was a male human. I'd heard bad things about GC, but took it anyway. So imagine my surprise to discover that Great Cleave was properly procing off-hand and doublestrike on all targets hit by the attack. Hooray!
After posting my find here, others chimed in saying that this was not the case for some other characters, such as female elves, who do a weird hop animation before GC that significantly slows the attack. What's more, on the characters that did the hop, they were reported to not proc off-hand attacks.
I found this very curious, so I did the most thorough test I could using freshly created level 7 characters with iTWF and GC. Here are the results:
In all cases where the hop animation is used for GC I never saw an off-hand attack (these characters didn't have enough doublestrike to test that). The races that use this animation are:
female humans and both sexes of Elf, Drow, and Half-orc.
The other races I tested did not use the hop animation, and GC properly procced off-hand attacks. These are:
male humans, and both sexes of Half-elves, halflings, dwarves, and warforged.
So that's the story, until something changes...