Ummm, not too many as a large amount of megas are quite big making their sprites really hard to make look decent.
But here we go:
WarGreymon, Omnimon, Imperialdramon (Dragon, Fighter and Paladin modes), MetalGarurumon, MaloMyotismon, Phoenixmon, MarineAngemon.
A few of them probably have no current way of getting, they are there though.
You may not have used a clean FireRed rom. Make sure it's English version 1.0. I use LunarIPS to do it, it works perfectly fine for me.
I just posted in the previous answer the currently inserted Megas. Use that if you want a Mega. There is Mega digivolutions, just not a whole bunch. We are all hard at work getting more sprites and just getting the digimon ready in general!