[csallen]

There are lots of examples on my site IndieHackers.com of people building up a side project from nothing. In general, they seem to do two things in the beginning.

First, they spend lots of time talking to everyone they can: friends, family, coworkers, people in their online communities, people at conferences, etc. This is not scalable in the long-run, but very helpful in the beginning to kick things off. Paul Graham has written about this extensively: http://paulgraham.com/ds.html

Second, the more successful founders find good distribution channels. They find app stores, online marketplaces, partnerships, etc that constantly put their product in front of lots potential customers. Easier said than done, of course, but the first step is just being aware of that approach.

[tedmiston]

There's an entire book written about the second approach called Traction [0] which boils down to: find the most dominant distribution channels and exploit them. Of course it includes plenty of non-traditional channel ideas and strategies as well.

[0]: http://tractionbook.com/