XeTeX 0.9999.3 for Mac was available to download from the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. XeTeX for Mac belongs to System Tools. The actual developer of this free software for Mac is Jonathan Kew.
From the developer:
XeTeX is Unicode-based TeX using platform fonts and font technologies (e.g. AAT on Mac OS X or OpenType features on Mac/Linux/Windows) to provide typesetting for all the world's scripts. With XeTeX, the advanced typographic features provided by OpenType fonts become available for all TEX users, as well as support for complex non-roman scripts.
You may want to check out more Mac applications, such as TeXworks, which might be similar to XeTeX.