Your best marketing idea that has helped spread the word about your product?
jmtame: Paul Buchheit at Startup School '08:Focus on making 100 people happy, and then you'll be validated for opening up your app to more people.I don't think it's that difficult getting 100 people (especially if you offer private beta invites to smaller niche sites that you're targeting).  Plus it gives you a chance to understand what your customers want, which is crucial to early development in an application.  You'll need to constantly get customer feedback and validation.