Current Stable API URL is always http://openlayers.org/api/OpenLayers.js
Current Version API URL is http://openlayers.org/api/2.13.1/OpenLayers.js
Older releases: http://openlayers.org/download/
Getting the Development Version From Github
OpenLayers' development branch is now on GitHub: http://github.com/openlayers/openlayers.
You can thus download OpenLayers by cloning the Git repository:
git clone https://github.com/openlayers/openlayers
The repository is world readable. Write access is limited. To contribute to OpenLayers' development, it's important to be comfortable working with git. The help pages on GitHub provide a nice starting place for this.
Or, if you prefer Subversion:
svn co https://github.com/openlayers/openlayers