Pro View Disassembler
Program-Transformation.Org: The Program Transformation Wiki
http://pvdasm.reverse-engineering.net/
ProView is a Pentium disassembler, PE (Windows Binary) editor, hex rditor, process manager,
and it has a Plugin SDK.
The whole tool is closed source, however the disassembly engine source code is freely available. A Windows binary (for the whole tool) is freely downloadable. Four (as of this writing) plugins are available for download.