Advertisers who buy their services. They pay spyware/adware writers an amount for information collected by spyware, or for each "hit" generated by adware (usually a few cents/hit).
OK - lets seperate them...
Spyware.... What information is collected?
Adware.... Is this where popups infect the machine and display actual ads randomly or sometimes related to keywords or something on the page? I've never had this happen but I know people who have reported it.