Cathode is a small program meant to make PNG-tubing as simple as possible, while being entirely cross-platform and open source. It takes a lot of inspiration from Veadotube-mini, a program with many of the same goals, but is made with the Unity engine and is currently closed source. Veadotube is great, and if you need something that’s ready for daily use, you should probably be using it.
But, if FOSS is important to you, or you don’t want to download 50 MB of Unity, then Cathode might be for you.