I have gotten most of my upsells from
AgeOath (off the ARS niche-specific FPA's, they work GREAT!),
CyberAge, and
FreeNetPass. The last one is a bit surprising to me, since the surfer bought a "free" membership (FNP), which is pretty easy to sell, but then they buy a paysite membership. Hmmm...
FNP and AgeOath are pretty easy to get on the link list with a fair bit of advertising; CA is a bit more strict. All three usually have decent traffic off the link list, depending on the niche of course.
The new AVS in my sig is also going to be good once they pump some traffic into it.
Bruce