What? The developer let the primary domain expire, but they still list it on their main site, which is still maintained:

And the Humble Bundle store still sells it here:

Not sure if the DRM-free direct download or torrent installer was exclusive to the old bundles it was included in, or if this store portal lets you acquire the same installers. Usually purchases from HB add to a list on your account with them, which lets you retrieve installers for all supported computer and sometimes mobile platforms, and sometimes included keys for stores like Steam instead of the original DRM-free downloads.