[osgovernment]

There is a C++ implementation that is pretty nice:

https://github.com/PurpleI2P/i2pd

[benji_is_me]

There was also Kovri (a fork of i2pd made by the Monero project) that sadly appears to be dead now.[1] It could have greatly increased the number of nodes on I2P.

[1]: https://gitlab.com/kovri-project/kovri