There are many services for this. Forums such as digital point or software auction sites like rentacoder work well. Even paying outright for your users is very common. Numerous large social sites with money have used affiliate systems such as commission junction, usually offering from .75$ to 2$ per user. Finally, youtube, myspace, every big social site has done contests that award people who refer the most new users.

I find the ways of getting users without paying for them directly to be more interesting though.