Hot on the heels of the Armv7a Gingerbread builds for B2G, we now have Mac and Windows Desktop builds of B2G+Gecko available for download. On TBPL, they show up as "Ng" cells in the existing OS X64 opt and Win opt rows, and you can download the builds from FTP in the nightly directories.
Please note that these are NOT full B2G builds. These are developer-targeted builds which only run on desktop machines, and cannot be flashed onto a phone or tablet. These builds are primarily useful to developers, QA and localizers working on Gaia. They can also be used by anyone who wishes to test out their websites or apps through a B2G-like client..
Linux versions of these builds are in the works, and will be available as soon as we fix an infrastructure issue preventing them from successfully building. We hope to have it fixed by the end of the week.
Edit: If you're confused about what to do with these builds even after they're running have a read over https://wiki.mozilla.org/Gaia/Hacking and https://wiki.mozilla.org/B2G/Hacking.