I got a request last week to do a lunchtime seminar of Monday on something different related to testing. After a bit of thought, I came up with Devel::TestEmbed, which embeds Test::More into the debugger so you can dynamically built test suites while debugging.
That's not so cool by itself, but it also creates a new tdump command that lets you dump out the tests you ran during the debugger session as a proper Test::More test, with the proper plan. It was almost as interesting developing the tests for the new command as it was implementing it.
The topper is that this is all done without actually changing any of the debugger's code at all!