Note: This functionality is now included with BrawlBox.
If you're looking for the site that converts .rel files for you, it's over here.
This version of the Stage Module Converter is written in Visual C#. Rather than relying on a directory containing all of the default .rel files, it allows the user to open stage modules (which might already be modified/converted) and change them to another stage. For Online Training Room .rel files, you can also change the item that spawns in the center of the stage.
The offset of the stage ID byte is found automatically by searching for the hex string 38 A5 00 00 38 80 00, while ignoring certain locations that are known to have "false positives" in NTSC-U or PAL versions of some module files. However, the offsets of the item ID bytes in the Online Training Room module files are hard-coded into the source.
To the extent possible under law, the author(s) have dedicated all copyright and related and neighboring rights to this software to the public domain worldwide. This software is distributed without any warranty.
You should have received a copy of the CC0 Public Domain Dedication along with this software. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
Download (C# source)