Mac OS X
Text version: Use the GUI version, and do a “Unison -> Install Command line tool”
GUI Universal Binary: 2.27.72 (2008.05.29, stable. This has been tested on Leopard intel and PPC. If something goes wrong, please drop me a line), 2.32.12 (2009.05.06, stable, should work on Tiger and Leopard), 2.40.1 (2010.02.02, beta, tested on 64 bits and 32 bits Snow Leopard), 2.40.16 (2010.06.29, beta, intel only)
Windows
Thanks to Karl M, here are native MSVC Windows version (GTK+, at least version 2.12, and text versions in each zip file): 2.27.157 (2009.06.30, stable), 2.32.52 (2009.06.30, stable), 2.40.16 (2010.04.21, beta).
Karl is using Pidgin GTK+ 2.14.7a for the 2.32 version. A runtime environment may be downloaded from the Pidgin project. If a gtk dll cannot be found (such as libgtk-win32-2-0.0.dll), it may be useful to add C:\Program Files\Common Files\GTK\2.0\bin to the PATH variable in the system settings.
You may also need to install the VC redistributable if it’s not present on your system.
Recent binaries may also be found on Jérôme Vouillon’s page.
Windows Text Only, Experimental version: 2.30.3 (2008.06.27), 2.30.6-cygwin (2008.07.01, requires the cygwin dll)
Linux
Thanks to Mike Elkevizth, here are Linux GTK+ version. 2.27.57 (2008.02.04, stable, compiled on CentOS 4.6), 2.32.52 64 bit (2009.07.01, stable, compiled on CentOS 5.3), 2.32.12 32 bit (2009.05.11, stable, compiled on CentOS 4.7).