1. Referrals, referrals, referrals. People loving the product has been by far the best way to promote it. I don't mean affiliate sales (where people are incented to refer), just the garden variety friend telling a friend kind. Nothing wrong with affiliates, of course, just a different thing.
2. SEO, to a smaller degree. When people say "SEO", I usually roll my eyes, cause it's often a non-answer. "Get lots of traffic by writing great viral content". Which is essentially a harder problem than promoting your web app. But having said that, at least from a search engine standpoint, it's something that pays dividends slowly, over time.
3. Banner ads/Google Ads - not particlarly effective, and something of a negative ROI investment, but this was good early on to get a feel for what kind of messaging worked best, and what sort of conversion rates were likely from direct ads. So I'd suggest using these to learn, not as a sustainable customer acquisition strategy, unless your price point supports it.
4. Press/bloggers - we got some writeups by a few bloggers, which generated some short term traffic (usually a week or so), then fell off dramatically. Again, good for SEO, and nice to get written about, but this hasn't been a consistent or reliable traffic source.
5. Web app directory/startup directory sites - good for short term launch traffic, generating awareness, etc., but these directories seem more noisy and less relevant over time.
Hope that helps.