Triconnectivity is a project mainly written in C++ and C, it's free.
C++ implementation of Hopcroft,Tarjan's algorithm and Schmidt's certifying algorithm for 3-connectivity testing using LEDA
This is an implementation of the algorithms of Hopcroft and Tarjan and Schmidt using LEDA, the Library of Efficient Datatypes and Algorithms.
The documentation in the code is somewhat lacking and the code is riddled with debug stuff, but you can read my master's thesis for a (hopefully) clean and understandable presentation.