FreeOrion numbered releases are available for download for Windows and OSX 10.6:
Older FreeOrion versions for Linux and OSX 10.5 are also available.
Please note that FreeOrion requires OpenGL 2.1 or later. With OpenGL less than 2.1, there may be poor performance or crashes.
Windows XP users may need to install the latest .NET framework to run FreeOrion.