Alpha, Beta and Release Candidate packages are built prior to the release of new major versions of PostgreSQL. They are not built continually.
Development snapshots are built from the latest source code and alpha/beta/rc versions that the developers are working on. These are 'bleeding edge' versions of PostgreSQL and should not be used in production systems as they have had little or no testing or quality control.
The latest development version of the documentation is also available online.
Alpha, Beta and Release Candidate tarballs are made available alongside the release tarballs during the final phases of the development cycle of new major versions of PostgreSQL.
Source code tarballs are built automatically every night on the official PostgreSQL development server. The development snapshot is taken from the head of the git repository, and includes the new features being worked on for the next release. There are also "branch tip" snapshots built from all supported stable branches, which contains all bugfixes that are scheduled for the next release.
Installers for Windows and Mac are available here (offsite link). These installers also include pgAdmin and are published by EnterpriseDB.
RPMs for Red Hat, CentOS, Fedora and Scientific Linux are available from the PostgreSQL Yum repository.
To setup the repository for these versions, follow the same instructions as for the normal yum repository on these platforms.
Typically only Beta and Release Candidate packages are available in the Yum repository.
DEBs for Debian and Ubuntu are available from the PostgreSQL APT Repository repository.
Typically only Beta and Release Candidate packages are available in the PostgreSQL APT repository.