At Amherst College, we are using the Wysiwyg module and had a need for a good HTML source editor so I modified the Wysiwyg module to add the Ace HTML editor as an editor. I figured I'd post it here in case it's useful to anyone.
I actually only added a few files, all separate and distinct from Wysiwyg so I didn't actually have to modify any of the Wysiwyg code, but I don't know a good way of adding the files I used and telling easily how to drop them into the existing Wysiwyg folder hierarchy. (for example, a diff wouldn't work because I'm not editing lines in the Wysiwyg module, just adding files)
Anyway, I just took our Wysiwyg directory (based on 6.x-2.4, the latest version), tarred it up, and attached it to this issue.
As for the version of Ace, this is tested off the latest in Github (so git clone git://github.com/ajaxorg/ace.git is the easiest way to get that) and it expects the library to be in sites/all/libraries in a folder called "ace". Should be self-explanatory and basically the same as the rest of the wysiwyg jazz and it also checks for the library as with any of the other wysiwyg libraries before allowing it to be enabled. This also expects jQuery UI to be present in sites/all/libraries. It also ignores basically all configuration options at the time being so if changing configuration options isn't working, that's basically expected behavior.
Disclaimer: This is unsupported. It works for us and I thought it may be useful so we're sharing as good members of the Drupal community ;) But if it doesn't work for you, then sorry, you're on your own. There is nothing we can do to help you, we cannot offer any support and will not respond to any requests for support. If anyone wants to take this and run with it, feel free to modify, tweak, etc. to your heart's content!
It would be nice to get some version of this committed back into the Wysiwyg module at some point, but we'd probably have to remove some of the dependencies first (like the dependency on jQuery UI).