Take a look at the Overlay Server from TortoiseHG:
it is Open Source, GPL2, and as far as I know does not rely on any none free software tools
implemented in a single file, TortoiseHgOverlayServer.py, of, at the moment, 553 lines including comments & blank lines.
Works with Windows XP through 10.
Very reliable but subject to the limitation that all such ...
I am using Dexpot right now, but it lacks some features & usability, since it mainly is a virtual desktop manager.
Notably the menu items are grouped in a sub menu, requiring an additional click to get things done with the system menu.
Also, "Minimize to Tray" does not "Minimize to Tray" when clicked, but instead sets up the window so that if I click ...
OwnCloud seems to have made their own implementation of icon overlays. It is available since 1.7.0.
The technical details of implementation are explained at https://github.com/owncloud/client/wiki/...
Liferay Nativity is a cross-platform library for adding icon overlays and context menus to file browsers.
Open source: Yes, LGPL
Reliable: It is used in a commercially-supported product, so I guess, but not tested
Communication is made via JSON/sockets, so it can presumably talk with C# after a bit of development on the C# side
Unfortunately seems to ...