Comment by BerislavLopac

This is a short list I have compiled a few months ago, FWIW:

    New kids on the block:
    - Starlette https://www.starlette.io
    - Vibora https://vibora.io
    - Xweb https://github.com/gaojiuli/xweb
    - Storm https://github.com/jiajunhuang/storm
    - Responder http://python-responder.org
    - Quart https://pgjones.gitlab.io/quart/
    - Sanic https://sanic.readthedocs.io
    - Bocadillo https://bocadilloproject.github.io/
    - Japronto https://github.com/squeaky-pl/japronto

    The old guard:
    - Flask http://flask.pocoo.org
    - Tornado http://www.tornadoweb.org
    - Falcon https://falconframework.org/
    - Bottle https://bottlepy.org
    - Pyramid https://trypyramid.com/
    - CherryPy https://cherrypy.org/

    Less known older ones:
    - Wheezy Web https://pythonhosted.org/wheezy.web/
    - API Hour https://pythonhosted.org/api_hour/
    - Morepath https://morepath.readthedocs.io/en/latest/toc.html
    - Klein https://github.com/twisted/klein
    - Weppy http://weppy.org/