It's interesting that reddit provides both good numbers, in _addition_ to a number of hard to quantify but tangible benefits.
HN
Reddit Ad
Craigslist
Friends & Family
Other suggestions are welcome :) (Not the side project in my profile)I would still recommend Twitter, Reddit, Delicious, and the like, but would also focus heavily on guest blogging nowadays - its effectiveness has been proven a lot in the last couple of years.
Edit: for something for consumers that is broad-based, like a search engine.
For example if you wanted to launch a site that listed the best places to rock climb, then you'd want to do some guest posts on popular rock climbing blogs.
I have not used them yet but I'm planning on it. I'm not sure I have found the down side at 5 bucks.