Isso is an open source alternative to Disqus. It is written in Python and allows users to enter comments in markdown. Unlike Disqus however, it is very lightweight and uses SQLite database store comments. Lately, Disqus has been criticized because of its privacy policies and bad site performance. If you are really concerned about these issues then isso might be a great alternative. This post describes how you can install isso on a blog written in Flask running on Ubuntu server though it can be adapted to any framework or server of your choice.