Where to upload a reprint (post-print) of my publication? I used to self-archive papers on my homepage in the institute where I was doing phd. I just noticed that my homepage and the reprints were removed -- that's normal, I'm not working in that institute anymore.

I can legally self-archive these papers. For example, J. Appl. Cryst. allows authors to post reprints


  on their own personal website, on their employer's website/repository
  and on free public servers in their subject area.


What are the best "free public servers" to upload electronic reprints?
(IIUC arXiv is for pre-prints rather than post-prints).
arXiv is not only for preprints (actually, updating after publication is highly encouraged). So if you can self-archive*) and you have an intersection with an arXiv discipline, it is the best place to place it. 

*) The only objection to post-publication is in case where you are not allowed to self-archive and don't wan't to mess with it.

For other disciplines, see Preprint services other than arXiv (for other fields).

Or (maybe even better) just - putting them on GitHub repository. It is a stable service + makes it easy to upload work with source code (tex + figures) (much alike arXiv, but on GitHub it is more convenient). Just make sure to link the pdf file in the README.md and reference it properly (i.e. the full citation, the best with a DOI link) so to have its status clear.