ntfy

ntfy (pronounce: notify) is a simple HTTP-based pub-sub notification service. It allows you to send notifications to your phone or desktop via scripts from any computer, entirely without signup or cost. It's also open source if you want to run your own. 

I run a free version of it at ntfy.sh. There's also an open source Android app (see Google Play or F-Droid), and an open source iOS app (see App Store).

Donations will primarily be used to cover the cost of running the free server (ntfy.sh), as well as to pay for developer licenses (Apple, Google). Currently the cost is about $400/year. Beyond that, donations of any kind will be humbly accepted and used for coffee and cookies. 

Disclaimer: In a past life, I've felt bad about taking breaks and such after receiving donations, which made me feel bad about my side project. So this time I thought I'd put it out there ahead of time: I'd love your donation to my project, but please do know that your donation does not mean that I'll fix bugs for you or solve your specific problems. I'll likely try to help out though, regardless of you donate or not.