I'd say one of the most common cuts for REAL MDMA is probably Brown Sugar, but obviously that will change drastically depending on the dealer. Most "MDMA" isn't MDMA however, and if that's the case then no one could know what it was until they had it tested.
From my experience: Methylone, Methylone and more Methylone. Some A-PVP and MDVP (bath salts). If you buying you're more likely to get amphetamines and piperazines. This is just the experience of one Canadian dude tho. BTW It's not just Canada that has a reputation for being cut, it's North America in general.
Drugs off the street can literally be cut with just about anything, or not even at all the drug being advertised. There are no rules in an unregulated market. I'm closing up this thread as it really serves no purpose beyond my response and it seems like there's a possibility a dealer could interpret this thread in a way to find out what cutting agents other dealers use so they can step on product. We're not here to help dealers step on their product.