X stands for extended. It is a monorepo of packages with similar functioniality to the standard library but with added convenience. Some of things you may find inside include cancelable IO, better signal based context cancelation, race runtime detection, prettier multierr, and more to come.
Most of these modules were previously hosted separately, but I have since decided to group them as a monorepo of modules. The previous versions will be archived, and the modules will start a new lifecycle here and start again from version v0.0.1.