Historical, stable version archive of packages
EDIT: This doesn't exist any more.
I'm actually working on an archive of CTAN using automated Mercurial repositories for the last two weeks. The CTAN content is mirrored daily and each CTAN package (with exceptions) is committed to its own Mercurial repository.
This is currently online as http://ctanhg.scharrer-online.de/, but not 100% finished yet. Also, it might not handle large loads well. You can get each archived version as ZIP or Mercurial clone.
The marginfix package entry is under http://ctanhg.scharrer-online.de/pkg/marginfix.html.
Update: I now manually added the older versions of this package. However, they weren't published on CTAN so I took them from the git repository. I only took the very first and last revision because they had versions attached to them.
All my own packages are accessible as normal Mercurial repositories under https://bitbucket.org/martin_scharrer/. These hold any commit I do, not just the CTAN releases. However, every released version is tagged and can therefore be retrieved easily.
CTAN holds the current release of each package: the 'archive' in the name is more because it's a single source for 'everything' than meant to imply a record. (Remember that before CTAN collecting TeX sources meant trawling many different author-maintained sites.)
Maintaining a 'back catalogue' of material is down to each package author, and as a result many packages have limited or no publicly-available source repository. The nearest you can get is probably the TeX Live SVN, which gets updated each time a package is changed in TeX Live. Of course, that means anything 'non-free' will be missing, and very occasionally will miss a short-lived version of a package. The later is most likely around the yearly TeX Live freeze.
there are only archives of complete TeXLive versions:
ftp://tug.org/historic/systems/texlive/