I interpreted this mostly for open-source projects trying to get (non-paying) users, because that's what I did. Maybe someone else will post about commercial projects.

• How to get up to 3500+ GitHub stars in one week
• How I Got From 0 to 1 000 Stars on GitHub in Three Months With My Open Source Side Project

All I personally did was:

• If someone specifically asked something that my library could do (e.g. on StackOverflow), I'd explain how to do it and post my library as an easy alternative at the end.
• Post it on my blog that no one reads.

I only got a little more than 50 stars per project, which I'm really happy with, but obviously isn't all that much.

There's also a monthly showcase topic on Hashnode.

It's all about marketing (as much as you said "not general tips").

All the things I'm suggesting also apply to startup development. I'd recommend checking out startup talks, particularly bootstrapping, since most side projects tend to fall under this category.

Intercom has a great article talking about getting their first customers that outlines a lot of the general topics I cover.

General tips:

• Word of mouth - Talk to people about your product. You are the biggest advocate for your project. Convince people why they should give you their valuable time to use your site/app. The relationships you create will be the foundation of your company. I spent weeks going through Tumblr and messaging people privately and politely about my blog to get my initial following.
• Build advocates - Allow people to become power users, or develop with, you project. The more integral you become to someone else, in terms of efficiency or profits, the more likely they are to care about your sustenance and success.
• SEO - Let your product grow organically by organizing it properly. If people have a problem, they tend to go to places like Google, and if Google can point people to your project -- it's doing all the acquisition for you. One of my side projects has never been advertised, but gets organic traffic globally from my SEO-optimized content.
• Build and empower your community - Either your site/app should be a community, or build a community (forums, Discord, Slack, Reddit, Gitter chat, etc) around your project. The people who really care about it should have a place to express their opinions about it, and invoke change. Passionate people usually want to contribute, you just have to lower the barrier of entry. Most side-projects usually have some sort of chatroom/group chat place to congregate and discuss the development and progression of it. On a lower level, companies use it as a place to build audiences looking for help with the app/library, or even as a place to meet like minded people and chill with good tunes.

Specific-ish tips

• Drop your app on a directory like ProductHunt
• Integrate other apps or services into your app. It helps justify other people's technology, and gets you on their radar.
• Find people with the problem your side project solves, and offer them the solution. Whether it's on StackOverflow, Twitter, or in person at events -- it's all about grassroots promotion in the beginning. "Are you using X package or app? Maybe consider mine because it's more efficient"
• Collaborate with content creators (bloggers, social media, video producers, etc) and have them expose people to your tech
• Create blog posts that talk about your product, on your personal blog and guest articles on other publications. Follow up those posts with a re-post on your social media accounts, Medium, Hashnode. Submit it to newsletters and directory listings.

Videos

• Great playlist on YouTube from YCombinator
• Curated talks For Startups by Startups