The current stable release line is 1.4. For more information about older versions, see the FAQ item about supported releases. Versions 1.2 and older should no longer be used due to known security issues that cannot be resolved in that code base.
Official release distribution
We generate the key with: gpg -bas [filename]
A Debian package is available in Debian Jessie (stable), Jessie-backports, Wheezy (oldstable) and Wheezy-backports.
Newer packages than those distributed by Debian, can be found in the LedgerSMB APT repository.
Ubuntu packages are available in the standard distributions as of Quantal (12.10) and Raring (13.04).
OpenBSD Packaging contributions solicited.
Virtual Box No up-to-date images at this time.
There are experimental Docker images available on the Docker hub. At this point the images reflect the main project, and are perfectly usable. The images are large (> 2GB) due to the LaTeX dependencies, and even with those, printing to PDF/Postscript is problematic especially if you want to customize the templates due to open issues with LaTeX include files (to load the common letterhead, for example).
Latest always points to the most recent stable release, which is currently the most recent 1.4.x release. The "dev-master" tag points to the most recent alpha or beta of the development 1.5.x version.
Keeping up with the latest developments
The latest 1.4.x and 1.5.x direct from GitHub as a .zip file (it's not an official release, but has the latest fixes you may need) 1.4.x from GitHub. Check the repository for the change log in 1.4.x from GitHub or the changes in bleeding edge ('master').