Here are some compiled versions of Unison. For more information about Unison, please see the project page or the Yahoo discussion group.

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)
GUI Intel: 2.40.61 (2011.01.28, stable), 2.40.69 (2011.07.19, stable, 64 bits only)
GUI PPC: 2.40.61 ppc (2011.01.28, stable, PPC only, thanks to Urs Mueller)

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), 2.40.102 (2013.01.22, stable).
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.

Bernhard Fisseni had the following tip regarding these binaries.
The GTK from Pidgin gives me errors about undefined symbols in libglib2. What works without further ado, however, is installing graphviz (http://www.graphviz.org), which also seems to bring its own GTK, and adds itself to the PATH.
In the previous quote, the undefined error message is the following:
The procedure entry point g_malloc_n could not be located in the dynamic
link library libglib-2.0.0.dll
An alternative to the installation of the whole graphviz application is the gtk-win project, which also includes the appropriate directory to the path (Thanks to Cameron Davidson for the tip).
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).

Thanks to Ivo Pletikosić, here is Linux GTK+ version 2.40.61 (2010.10.26, stable, 32 bits).

Thanks to Urs Mueller, here are text only static versions 2.40.61 32 bits (2011.01.28, stable, 32 bits), 2.40.61 64 bits (2011.01.28, stable, 64 bits).

Thanks to Lyle Gordon, here is an Oracle Solaris 11 Express version, compiled with ocaml 3.10.2 and lablgtk 2.10.1: 2.40.63 (2011.05.03, stable)

Thanks to Denoyse, here is an arm version, compiled under Debian squeeze on a Marvell Sheeva CPU (DreamPlug): 2.40.63 (2011.10.16, stable)

Thanks to Greg Huber, here is a a centos 5.7 x86_64 version: 2.40.63 (2012.01.20, stable)

Jens Wagner has compiled a static ARM binary for unison 2.40.102. He is hosting it, as well as instruction for compilation, on his web site.