Winpdb Reborn is one of the most important and well-known Python debuggers. The major strength of this debugger is managing the debugging of thread-based code.
Winpdb Reborn is based on the RPDB2 debugger, while Winpdb is the GUI frontend to RPDB2 (see: https://github.com/bluebird75/winpdb/blob/master/rpdb2.py).