For starters, there is main tor implementation, the one in C.
There is also a Java version of Tor in development, called JTor, however it has not be 'released' yet.
Finally, there is an implementation of Tor written in python using the Scapy library called TorPylle, but it is an experimental implementation more to explain the protocol and should not be used.
There also exist number of projects that use tor as backend, but do not itself implement it (they might bundle tor, but do not maintain their own source code). There is a list of projects that The Tor Project maintains.