Your word of mouth... Once people arrive (if it is worwhile), they will tell others and it can continue.
Both typlo and jhonson5 have it right. They all require maintenance in order to continue your growth, but it really does rely on the website. If it is 'cool' or the 'best' in it's category online, then people will help it become popular. By the same token, if visitors arrive and it is 'crap' then that is what they carry away with them... the same applies for linking, etc.
#1 by far, is search engine traffic. These are qualified individuals, not just eyeballs. These people "looked-up" exactly what you have. The rest is up to your pages.
You can pay for massive traffic and still not reach any notoriety or even sales increase... because your visitor is of lower quality. Search engine traffic continues for free and could increase over time. Own your category online (or be one of the top 5 websites that people would bookmark) If your website or url is the same ol' same 'ol then just getting hits is tough, but getting quality visitors on a consistent basis is impossible. This is all of course, in my opinion.