There is no stable release of Calligra yet, but stable versions of the application can be found in KOffice which is the predecessor of Calligra. For downloads of KOffice see the KOffice Download page.
Usage of unstable version is not recommended by the Calligra team.
Packages are available from the unstable playground repository.
Ubuntu users can get a nightly build of Calligra with the Project Neon, you need to add ppa:neon/ppa to your sources list and install project-neon-calligra package.