I stumbled upon a treasure trove of Mac OS X XML oriented software the other day. All of which are free, some open source, and all very well put together.

They are made by a guy named Todd Ditchendorf who, apparently, used to work at Apple.

Some helpful gems that I use and recommend:

His main project appears to be a project called Fluid which from the demo seems like AjaxLocal done correctly (think Adobe AIR without the local access / flash part).