I have a lot of ideas for small interesting websites like this, but I've no clue how to popularize them at all. Maybe a Show HN or something, but that seems like a short burst if the site is more of a novelty.
In first year, I maybe leaved link on one or two subreddits, with not too many subscribers. I had little bit of traffic from reddit.
I had this url also in signature in 2 discussion boards, but boards were in Croatian language. So only little bit of traffic from there.
Then after some time I got better Google ranking. (there was no too much similiar sites there, so it was easy to get better ranking).
Once someone on 4chan or similair chan site posted link.
Basically, it has grown without marketing/advertising. But still, it is small site.