Richpost is an extension which allows you to read and write MarkDown in Google Plus. It relies on the post having a #richpost tag through which it then recognizes convertible posts.
It's still very much a work in progress and an open todo list can be found at the github repo (https://github.com/Swader/richpost) if you feel like contributing.
For a detailed writeup on how and why this was built, see the blog post: http://www.bitfalls.com/2013/09/markdown-support-in-google-plus-chrome.html
Extore is a team of professionals who are passionate about creating extensions for web browsers. This devotion also gives us opportunity to appreciate work of other people. We get inspired by useful, open source extensions made by developers all over the world. Our strong belief is that one should share helpful add-ons with others. That’s why we’d like to present you our ever-growing list of favourite extensions that have inspired us. Moreover, as you probably have already learned, sometimes struggling for the best brings an end to the good. Talking about add-ons it often turns out that an upgrade is worse then previous version. Also it can become really tricky to get back to the beloved version of your favourite extension. That’s why we’re going to make not only the latest, but all versions of our favourite (and we hope yours too) add-ons available for download. No more compromises, just stick to the version you really like!