Google and various Twitter application directories.
Do you spend any money on getting users?
None so far, but am looking to change that in the summer. I'm getting married in a couple of weeks, and then honeymoonining, so things are getting a bit hectic, but once those have calmed down I'm thinking of experimenting with Adwords :)
Congratulations, and thanks for the inspiring article. I have had an idea on the go on and off for a while now and this is inspiring me to get off my butt and finish it.
What's your most important sources of traffic?
Do you spend any money on getting users?