I only know of commercial ones - it's a pity - and it sort of annoys me where everyone just wants to make money out of everything without offering something free to the community.
I personally don't use the free forums as I found they were a bit buggy for me.
However, a few years ago, i worked with Chris Paterra to make the new forum layouts and icons. They have been superseded and updated since then, but here is how I started.
I installed the forums locally, but if you don't have a local build, then live would do.
I downloaded the forums folder and backed it up, zipping it as my master.
I changed the images, and that's the tedious part - it takes ages to get those new icons looking as you want but fortunately there are many many websites with icons for download.
I then modified the css file and at the time, changing themes didn't work as well as expected, but I believe it is better now.
I then uploaded the files and overwrote them, and saw the changes, if i liked, I kept, if not, i'd go back in and make more changes.
Sometimes I'd zip up with the name/time on there so I could go back to it if i mucked things up.
So, while it doesn't help you right now, that is the procedure I used when on the core team and making these forum changes. it was harder then because at the time we were trying to untangle hard coded elements so that it could be completely template driven. That was quite a few years ago and I believe that it's now much easier.