These are built from the current development sources (Git master branch).
To set up file type associations and AutoPlay handlers, use mpv-install.bat.
To play content from various media streaming sites, place youtube-dl.exe in the same directory as mpv. I recommend updating it together with mpv. You might want to use this with a browser extension that can open URLs with external applications (Firefox, Chrome).
If you notice anything wrong with these builds, complain to lachs0r on Freenode or report the problem on the issue tracker.
Be advised that the SMPlayer frontend is not supported, and that any claims that it has proper support for mpv are outright lies.
I was maintaining SMPlayer2 back in the mplayer2 days, so I’m familiar enough with SMPlayer’s development practices and awful code quality (which have not changed since then) to make this assertion. Fixing SMPlayer would amount to a rewrite, which is not very likely to happen.