Pushover is a nice tool to add push-notifications to your private projects with a little one-time cost - or to your business applications for a monthly per user fee. It has a straightforward API with nearly everything you need to notify you or other users. I started using it years ago when starting to create my Smarthome and am still happy. Also found useful if you want to build prototypes that have notification features as it saves you tons of set up hassle...