The current stable Psycopg release is 2.5.1:
There is currently no testing release.
See the install page for instructions about how to install Psycopg from source.
Jason Erickson maintains a packaged Windows port of Psycopg.
If you want to build on Windows yourself, here is an article with some tips.
Mac OS X Packages
Psycopg is available as a fink package in the unstable tree: it should be possible to install it with:
fink install psycopg2-py27
The package is also available on MacPorts: try:
sudo port install py27-psycopg2
If you want to build Psycopg from source, take a look at these articles.
All the Psycopg releases (both branches 2.0 and the now obsolete 1.0) are available for download.