There are a great many other applications and projects working on anonymous communication and I2P has been inspired by much of their efforts. This is not a comprehensive list of anonymity resources - both freehaven's Anonymity Bibliography and GNUnet's related projects serve that purpose well. That said, a few systems stand out for further comparison:
Morphmix and Tarzan are both fully distributed, peer to peer networks of anonymizing proxies, allowing people to tunnel out through the low latency mix network. Morphmix includes some very interesting collusion detection algorithms and Sybil defenses, while Tarzan makes use of the scarcity of IP addresses to accomplishs the same. The two primary differences between these systems and I2P are related to I2P's threat model and their out-proxy design (as opposed to providing both sender and receiver anonymity). There is source code available to both systems, but we are not aware of their use outside of academic environments.
Stealing quite directly from the Tarzan paper, the following includes a quick comparison of Tarzan, Crowds, Onion Routing (OR), and I2P:
TOR and Onion Routing are both anonymizing proxy networks, allowing people to tunnel out through their low latency mix network. The two primary differences between TOR / OnionRouting and I2P are again related to differences in the threat model and the out-proxy design (though TOR is working to provide redevous points within the mix network, which will provide recipient anonymity). In addition, these networks take the directory based approach - providing a centralized point to manage the overall 'view' of the network, as well as gather and report statistics, as opposed to I2P's distributed network database and peer selection.
On the technical side, there are 5 main differences between TOR and I2P:
Mixminion and Mixmaster are networks to support anonymous email against a very powerful adversary. I2P aims to provide an adequate means to meet their threat model as we reach I2P 3.0 along side the needs of low latency users, providing a significantly larger anonymity set. As with TOR and Onion Routing above, both Mixminion and Mixmaster take the directory based approach as well.
Freenet is a fully distributed, peer to peer anonymous publishing network. As such, generic anonymous communication over it requires the use of the global blackboard model - storing data somewhere that the recipient will then check for a message. Freenet also does not support the concept of user defined delays - it stores and fetches data as quickly as it can, rather than queueing up, pooling, delaying, and mixing the data, leaving a hole with regards to long term intersection attacks. In addition, there seem to be some performance issues that can arguably be attributed to the global blackboard model which will likely rule out interactive low latency communication.
JAP (Java Anonymous Proxy) is a network of mix cascades for anonymizing web requests, and as such it has a few centralized nodes (participants in the cascade) that blend and mix requests from clients through the sequence of nodes (the cascade) before proxying out onto the web. The scope, threat model, and security is substantially different from I2P, but for those who don't require significant anonymity but still are not satisfied with an Anonymizer-like service, JAP is worth reviewing. One caution to note is that anyone under the jurisdiction of the German courts may want to take care, as the German Federal Bureau of Criminal Investigation (FBCI) has has successfully mounted an [attack] on the network. Even though the method of this attack was later found to be illegal in the German courts, the fact that the data was successfully collected is the concern. Courts change their minds based upon circumstance, and this is evidence that if a government body or intelligence agency wanted to, they could gather the data, even if it may be found inadmissible in some courts later)