I'm looking for a simulation of the Quantum Key Distribution protocol. Does anyone here have one that they could recommend?
There are several kind of quantum key distribution (QKD) protocols as of today. Are you looking for a particular one?
The best known QKD protocol goes by the name BB84 after its inventors Bennett and Brassard and the year in which they presented their work. Searching on the Internet, I found this link http://fredhenle.net/bb84/demo.php with a simulation that looks fairly nice and introductory.
As such, programming (at least) an ideal version of the BB84 protocol is not difficult. By ideal I mean that Alice's source, Bob's receiver are perfect, and the channel is not only without Eve but also does not absorb or distort the information carriers (in case you do not have any background in quantum physics, you could also read section 2 of this paper http://arxiv.org/abs/1206.7019 that serves to explain the operation of BB84 to even non-scientists).
Otherwise, with the following steps you can simulate BB84 on your own:
Actually recently I found out about a complete QKD simulation toolkit that has become available, accessible online via this link, QKD simulator.
It is a parameter-based simulator, so different scenarios (qubit numbers, Eve's influence, etc.) can be set up and simulated.