The scope of this particular list is limited to things that infrastructure developers (System Administrator, DevOps Practitioners, etc.) are likely to find useful. We love all the free services out there, but it would be good to keep it on topic. It's a bit of a grey line at times so this is a bit opinionated; do not be offended if I do not accept your contribution.
This list is the result of Pull Requests, reviews, ideas and work done by 900+ people. You too can help by sending Pull Requests to add more services or by remove ones whose offerings have changed or been retired.