The CALF Plugins are pretty excellent, for the most part. Those are free (as in beer and as in speech, IIRC).
Those were the main ones I worked with, plus another commercial package, I forgot its name, which was decent.
The Calf compressor and bass enhancer were very good. I'm seeing that a lot of plugins have been added in the time since I last worked with Ardour. Makes me want to start making music again :)
Nothing like knocking the rust off :-)
Unfortunately, the CALF series have been notorious for having problems or causing crashes in the past. If you do want to try it out, I highly recommend getting the latest version or building from git as a lot of distributions package an older version in their repositories.
I actually didn't experience that issue.
Things are slowly getting better if you want to run Windows VSTs under Linux. I found this very interesting as a overview of some of the current possibilities: https://www.youtube.com/watch?v=jsZnSg4DKDU
It is a demonstration for a proprietary DAW, Bitwig, but he does talk a lot about Windows plugins under Linux that I imagine applies to Ardour as well.
The main problem was getting high quality effects/plugins, paid or free. There are a couple, but in general life is much easier in this sense outside of Linux. That was 2-3 years ago, and hopefully things have improved since.