Biggest bang for your buck?

I'm a developer, I've built my side project and now I need to let people know about it, but wherever I look I have to spend a lot of time engaging with each community to get any traction and without much idea what the cost (time) / benefit will be.

Don't get me wrong... I hate the fly-by spam postings for "Buy my super cool product" as much as the next person but on a limited timescale (4-8 hours a week which is divided between development, bug fixes, SEO, R&D, marketing etc) there is only much engagement that can be realistically achieved.

So my question is, what are your go-to networks/groups etc? What are the ones that immediately pop into your head when you think about sending an update about new features? Where are you happy to engage with the community and you feel there is a reasonable cost/benefit tradeoff?