Synapsen " is a hypertextual card index or reference organizer, which is to say a storage system on the basis of an electronic literature database that enables the management of bibliographies ... synapsen offers a very distinct advantage: with catchwords [keywords, MK] entered by the user, the program connects individual cards automatically and creates not only a network of cards whose relation to each other might have been forgotten, but also creates completely unexpected connections and relations between individual entries." It is in some ways similar to Daniel Lüdecke's Zettelkasten, which has seen much more development during the last few years, however.
Synapsen is also described by the developer as "a hypertextual card catalog or reference management software written in JAVA." It uses an a "SQL-database based in [sic] the literature networking structure [sic] from [sic] Niklas Luhmann." As I have said before, Luhmann's Zettelkasten did not primarily (nor even secondarily) rely on connections by means of keywords. It was based on direct connections of Zettel or index cards to one another (based on a numbering scheme). It was thus much more similar to a Wiki, like ConnectedText.
1. It now allows direct manual links between entries, by the way.