FreeOrion is available as precompiled point releases for Windows XP, statically-linked Linux binaries, and a MacOSX universal binary. The source code may also be downloaded and compiled for Linux, WinXP or MacOSX.
Please note that FreeOrion requires OpenGL 2.0 or later.
With OpenGL support less than 2.0, the program might run and show the introduction / title screen, but crash when starting a game, or might require some rendering options to be disabled, or might crash immediately on startup, depending on your particular graphics hardware and drivers.
Windows XP, Vista and 7
Installable Point Releases
Point releases for 32-bit Windows XP, Vista or Windows 7 can be downloaded from SourceForge.
Please note that on Windows XP, The Windows XP Service Pack 2 and the .NET Framework 2.0 (or later) are required to run FreeOrion. FreeOrion doesn't use any .NET code or features, but the Visual Studio compiler adds this dependency regardless.
The .NET Framework 2.0 can be installed by by running Windows Update, selecting "Custom", then "Optional, Software", then ".NET Framework 2.0". Alternatively, it may be downloaded from Microsoft directly. More recent .NET Framework versions should also work.
Compiled Binaries Only
Zip files containing just the compiled executables of FreeOrion are available on the forums in this thread.
As noted in that thread, to run these executables, it will be necessary to download the FreeOrion Windows SDK, and possibly to update the data files with the latest versions from SVN. If you are unable or unwilling to do this, packaged installers ("point releases" discussed below) are made available less frequently.
Statically linked binaries are available in this forum thread, and may be avaialble from SourceForge for the latest FreeOrion point release. Generally the versions in the forum thread are more up-to-date, but numbered releases on SourceForge more closely match the content of the Windows releases.
These binaries have been tested on Ubuntu 9.04 Jaunty, Ubuntu 8.10, Debian Lenny, Debian 4.0 Etch and Debian Sid, and may work on other distributions as well. They should also run on x86_64 linux if ia32 libs are installed. OpenGL 2.0 or later is required and without it, running freeorion may cause segmentation faults.
RPM packages are available for FreeOrion v0.3.11 (SVN revision 2732) for Mandriva Linux. See this forum post for a link.