I started using Buster[1] few months back with Google Speech to Text API. I use Firefox on Linux and I hadn't had to manually fill a captcha in all these months. Highly recommended. :)
I get 60 free minutes/month from Google Speech to text API. So, It's pretty much free to use.
[1]: https://addons.mozilla.org/en-US/firefox/addon/buster-captch...