Userland Frontier Kernel
As of 2014-01-04 the master branch will run on macOS 10.12 (It will not build on > 10.10. It currently builds against the 10.6 SDK, but still relies on deprecated frameworks.)
yosemite branch builds and runs on macOS 10.12.
You can download a binary of the OPML Editor built from the master tree here: Download OPML Editor v10.1b21.