Syndie is being developed in part by the I2P development team, keeping in mind both I2P's anonymity and security model so as to support trivial operation over the I2P network. I2P by itself simply transfers small (1-4KB) messages between anonymous "destinations", while Syndie works at a higher layer, coordinating what content gets transferred in the first place, as well as allowing end user interaction.
Tor (up)As with I2P, Tor is a low latency anonymizing network, and Tor's requirements and capabilities have been kept in mind so as to allow Syndie to operate on top of Tor as well.
Freenet (up)Freenet is an anonymizing distributed data cache whose features complement Syndie as well. Inserting a Syndie archive into Freenet is not a problem, and syndicating off Freenet SSK or USK keys is trivial, given Freenet's HTTP interface.
Frost (up)Frost, one of the most popular Freenet applications, bears many similarities to Syndie. The most substantial difference is Frost's tight dependence upon Freenet, limiting the flexibility of the message boards, constraining the potential user base to a subset of Freenet's users, and tying its viability to that of Freenet as a whole. There are of course other important differences, such as Frost's functionality as a file sharing interface to Freenet and Syndie's ability to hide both the pseudonyms and even the quantity of messages being posted in a forum.
OpenDHT (up)Another public storage medium that Syndie can be made to operate archives out of. OpenDHT runs across hundreds of geographically and politically dispersed servers, hosted by PlanetLab.
Eternity Service (up)Syndie has similarities with Ross Anderson's Eternity Service and Adam Back's subsequent Eternity USENET, but with substantial differences. For instance, rather than integrate the content layer with the mixnet and a payment system (as the original Eternity Service did), Syndie operates on top of the various mixnets available today, and leaves payment out of the scope. Syndie also differs from the various Eternity Service implementations in that Syndie is not meant to make a piece of content available to anyone forever, though a subset of Syndie's users may choose to run it in that way. Syndie perhaps could be seen as a way to build many separate (crypto-enhanced) Usenet networks, each with their own content management policy, distribution mechanism, and namespace.