Not yet mentioned yet is http://lucida.ai/ -- it's the successor to Sirius, and where all the ongoing development is focused.
Major options that are mentioned elsewhere in the thread:
https://mycroft.ai/ (One of the most advanced,can actually be used in a pretty useful manner now, but sends snippets to Google for voice recognition--they intend to change that eventually, and they don't have a full-time open mic. Plus they aggregate audio across users so it's less identifiable as from a single source).
https://wiki.mozilla.org/Vaani (from the Mozilla project; supposed to enter beta this month according to that page)