For various reasons, we have chosen not to distribute pre-compiled binary versions of MythTV. However, if you would like to compile from source, you may download the latest versions here:
If you are looking for older versions, you can find them in the OSU OSL archives.
If you would like to forgo compiling MythTV from source, there are packages available for a variety of linux distrubutions. You can find a list of available third party packages in the wiki, along with a handful of howtos for getting MythTV installed in your favorite distro.
In the wiki, you will also find a list of full-blown linux distributions that come pre-installed with MythTV, along with howtos for getting MythTV running on most other major linux distributions (as well as FreeBSD).
There are also pre-built versions of MythTV for MacOS available from several different websites.
There are no known pre-built versions of MythTV for Windows yet. Until then, you'll have to build it yourself by following the instructions. Hopefully someone will eventually offer a pre-built version and put a link on that page.
If you would like to grab the current development version or just want to keep up with the latest fixes to the stable branch, you will find git checkout instructions at https://github.com/MythTV