The Online Card and Tabletop Gaming Network (OCTGN) is a Game engine for playing card and tabletop games online. It's an application for MS-Windows and run on .Net4.
Anyone can create a game definition file (A mix of xml and python) which will define the various rules of the game and any scripts to automate various play. Players then import the various sets of the card game and create decks to play. It varies from most other online card game engines, such as LackeyCCG, with its implementation of the Python language for advanced game scripting, which allows each game plugin's author to automate various tedious tasks to speed up the game or avoid player mistakes.
OCTGN is currently in its 3rd major version and it features a built-in chat lobby and game hosting server. It is actively developed and maintained as an open-source project with a funding plan to increase server capability and provide a fully cross-platform experience via a planned 4th major version release.
One can find some available games and sets in the OCTGN Game Directory.
The Munchkin plugin is available here