One place you can check is on Github, these two accounts have a lot of plugins:
Note, there are no guarantees that every repo there is in a good stable state.
Aside from that, there was a list on the Q2A wiki, but it's not really in a good state and is quite outdated.
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.