It depends on the product.

Fleaflicker's sole source of promotion the first two seasons was Google AdWords. Fantasy football search terms are always popular; one user would invite a dozen of their friends to play after they discovered the site.

I tried other methods but that provided by far the best return.

I spent a lot of time E-mailing bloggers and journalists. The reply rate is very low but if one person actually reads your message writes about the site, the payoff is huge. Obviously don't spam when you do this. Research the journalist you're pitching to and make your E-mail memorable. It's kind of like a cover letter when applying for a job.