If you install this userscript, 5% of the money you spend at Amazon or libri.de, actually goes to FSFE, to support the struggle for Software Freedom!
Of course it is Free Software. It also protects your privacy a tiny bit better, by defaulting to encrypted connections with the vendors’ sites.
Some online shops offer affialiate programs, where if it looks like an affiliate partner made someone buy at that online shop the affiliate partner receives part of the profits. Currently FSFE is taking part in this program with the different Amazons and also with libri.de.
Note that neither myself, nor FSFE want to promote either of the vendors, the userscript in no way advertises anything, it is just there, so that if you would buy there anyway, some of the money goes to a good/better cause.
See http://wiki.fsfe.org/SupportPrograms for more information.
After reading some other userscripts I wrote one that adds FSFE-affiliate links to Amazon products (depending on which Amazon-service you are using) and also switches links from pointing to .libri.de to .bookzilla.de (the FSFE affiliate program with libri.de).
These changes are made both on the sites and with links pointing to these site. The script also always uses SSL and shortens product links on amazon to be more easily readable (and distributable).