Zero advertising. All I did was link it from my blog and a few other websites I own and tweet about it a few times. I imagine the strength of the domain name and the keywords on the page help a lot. It's also been linked as a useful tool on many highly trafficked and well respected blogs, so it's got some very good credible link juice going for it.

Honestly I just created it for myself back before Grunt, Gulp, etc. after searching for some simple online tools and not finding any that allowed uploading and combining multiple files together. The first version was PHP, and I switched it to a node.js app in 2011 after Uglify.js came out (there were a lot of problems with the PHP library I was using).

I created the website in 2008, so it's been going for quite a while, collecting backlinks and traffic ever since. I am not looking at the reports right now, but I believe it made less than $200 the whole first year I had ads on it (it didn't always have ads).