Packages are built in the openSUSE Build Service for a variety of Linux distributions. You can find all of the packages in home:/mbarringer or use the "1 Click Install" links to automate the process a bit. Not all packages are tested on all distributions for each version, so if a package doesn't work for you, let us know.
You can find the latest Windows installer at Sourceforge. It should run on XP, Vista, and Windows 7.
A disk image for OS X is available at Sourceforge. It has been tested on 10.5 and 10.6.
git clone https://github.com/Entomologist/entomologist.git(keep in mind that the master branch may not be stable!)
make. You'll need Qt >= 4.6.