Can’t say that I pay for promotion but here are a few examples I can think of that people might be paying for.
SEO Optimization (alotta do it yourselfers here, but if that isn’t you, I’m pretty sure you can find someone to do it for ya)
Forum posting (I consider this promoting.. noone wants to talk to themselves in an empty forum)
Links on high PR pages (Lotsa people here sell links to help others out of the sandbox)
PPC Search engine (not the best plan in my book, but ppl pay for it)
Contests (I’ve done $25 for best written review, and have participated in a few other webmasters contests myself)
That’s about all I can think about for now.. anyone else?