Bash tooling for version management of lots of programs. In some cases it wraps exisitng tools (python uses pyevn, ruby uses rbenv) but its all plugin based and super helpful for quickly switching versions of apps like terraform or documenting versions via its .tool-versions file which works something like direnv. And you can just run

asdf install

in a directory with a tool versions file and it will Do The Right Thing. Really great ecosystem. Very useful tool.