is a program built according to Niklas Luhmann's slip box system. With the Zettelkasten, you can organize your excerpts, abstracts or scientific quotes in a similar way like Luhmann did. Instead of relying to categories, the Zettelkastens uses so called keywords
(tags) to tag and link entries, which helps you to build up a network of entry-connections and leads to relations between thoughts that haven't been anticipated in such form - thus leading to unexpected, suprising ideas and conclusions.
The Zettelkasten is free
for personal or job use and available for Windows, Mac OS X and Linux.
Currently there's only a German version available, but localizations of the programmes are in progress... If you are interested, you can participate and help with the translation
of the Zettelkasten.