A few weeks back I did the work to make most of the FreeBSD regression test suite produce output that's compatible with Test::Harness and prove(1).
Back then there wasn't a name for that format. Now, of course, there is.
This was very much a rough cut. A lot of the tests were written in C, consisting of a few hundred (or in some cases a few thousand) ASSERT() calls, with a printf("PASS\n"); at the bottom -- if your test didn't dump core you knew it worked.
Changing most of those to add printf("1..1\n"); and printf("ok 1\n"); was the work of an afternoon.
On the TODO list was to implement as much of TAP as possible in C -- having plans, ok(), and so on. By doing that, and removing the ASSERT()s, a failing test doesn't prevent all the other tests from running, and you get a truer picture about how many tests overall are being run. And we all know that seeing the test count go up is a strong incentive to keep writing them.
So, here's a very early draft of that; libtap. Right now, plan(), ok(), and diag() functionality is there, which is almost all the way to what I need for FreeBSD regression suite.