I posted on HN before heading to lunch (at my law firm) on day. Got tons of users that way (but not as many as I'd have gotten if I had an email signup form on the site...doh). I later learned it was the 9th-most-popular Show HN post ever. [1]

I also got some good traction posting on a couple subreddits, but I learned that you have to be very careful abiding by the rules about posting about your own stuff, and other details. After a couple posts with amazing trajectories that were shot down, I stopped even trying. I know some people have had more success by convincing their users to post there, which sidesteps much of the potential drama around self-promotion.

1: https://news.ycombinator.com/item?id=6335784