https://destructible.io for sharing files temporarily with yourself across terminals, or others. I submitted to HN, got a huge spike of subscribers. Submitted to Reddit, got another, smaller spike.

Eventually other people started submitting it in response to questions online in forums like HN, where people would ask things like, "What's the Best Productivity Tool You've Found" or, "What Secret Thing Do You Wish Everyone Knew About." Started getting more spikes in users, then a regular base of users, then paying customers, then enterprise customers! Pretty cool organic spread.

It's definitely a side project, but making some money on it which is awesome.

Tried Google AdWords, total waste of (free up to coupon amount) money. Maybe one new user with $250 spent.

So I would say: Build a good product and maybe if people will like it enough you will get some organic growth.

edit to say: I'm still 1000% just doing this as a fun aide project that I built to serve a need I specifically had, but am happy to answer specific questions about what I stumbled through and did to kind of get off the ground enough to pay monthly costs and make a little profit.