for the free servers, there's a deal called double everything (you can find it through link in cpanel). The offer can be used twice, so you get double double everything. What you end up with depends on what package you got to start with, ie ad-free or ad-enhanced. ($4.95 x2 if you want the double double deal, one time payment).
I know there used to be a deal that didn't double eveything, but can't find a link. Think that one was space and possibly bandwidth only. The double everything doubles ftp accounts, add on domains etcetera.