This project has .NET and Java libraries for writing CZML content for use with Cesium. For a python library, see czml by @cleder.
See the CZML specification for more information on the structure of CZML.
Apache 2.0. czml-writer is free for both commercial and non-commercial use.
Binary releases are available on NuGet.
Check out the wiki, including the quick start and contributor's guide. Also browse the forum.