For all those people out there who lease/rent a dedicated server, what do you use it for? Reselling web accounts? Game server? Adult site?

It really depends. Most use it for hosting clients. I've seen a few being used as game servers, you can't really run that many game servers since the engines use up a lot of CPU/memory. Other people just use it to play around with, such as learning to use linux better.

Gaming servers, when I feel in the gaming mood.
Mostly resellers and hosting.

personal use mostly (i have 8 domains myself) and my partner has another dozen or so... then we run hosting on the side to pay for the server. :)

I use one for an email server only, and I host about 25 sites on another. I like to keep a seperate mail server running.