First of all let me say that I am biased as I work on WPM, but I'll try to be as fair minded as possible.
In the table below is some data about Windows package managers. Some of them are listed by Wikipedia, some are mentioned in answers to this StackOverflow question.
I do not try to compare the software itself, either GUI or command line (maybe next time). I only try to compare the default repositories. The number of available packages does not include different versions of a package.
I did not include application-level package management solutions like PEAR or Maven as they are a different story and are not useful for everybody. I also removed the Windows Installer and some others because there is no central repository and Cygwin because it only works for the packages defined there. There are also some sites like ninite that let you download a package consisting of different useful software at once. They were also not included in this comparison because they are not flexible enough to be called a package manager (unattended installation and un-installation are missing).
So here is the current state of package repositories for Windows:
| Software | Number of available packages | Number of package versions published in August 2010 | Package formats |
|---|---|---|---|
| Appsnap | 379 | 0 (3) | any |
| Appupdater | 84 | ? (2) | any |
| GetIt | 216 (1) | ? (2) | any |
| Windows-get | 243 | 0 | any |
| WPM | 143 | 28 | any |
(1) GetIt does not have an own repository, but uses that of Appupdater, Appsnap and Win-Get
(2) no information available
(3) although the repository was not updated for a long period of time, most packages should be up-to-date as the software uses rules to find the newest package versions
