Botnets have been used for email spamming for a long time. They are increasingly used for other purposes such as click fraud. What are the typical clients of a Botnet operator?
Also, have you heard about open Botnets, that is, Botnets in which participants (typically owning or managing clustered IP addresses) agree to share their servers for the community? In short, a Botnet in which all "victims" have agreed to participate in the scheme, and have (knowingly) their computers infected, and benefiting from the scheme.