RMX is Twitter shorthand for remix.
It's a retweet (RT) where you do a mashup on the original tweet adding your own opinions.
The original twitterer @aaamplo writes:
watching Wolverine trailer http:\\fakelink
So a regular RT would be:
RT @aaamplo: watching Wolverine trailer http:\\fakelink
But an RMX would be like:
RMX @aaamplo: Wolverine trailer http:\\fakelink sux vs. Origins http:\\fakelink2
Otherwise known as the "remix"
Ludacris - Put your money (rmx)