At work we have these "do-hickeys" that look like a regular "tee" connector.
However, it's meant to be used such that a signal passes through the "top"
of the tee while the "leg" of the tee "picks off" the signal some 10dB or so
down from the input. (The insertion loss is some fraction of a dB.) The
pick off seems to just be a wire tip coming out of the backside of the
connector (just capacitively coupled?); there's an adjustable sleeve that
lets you position this tip closer or further from the through line; moving
it closer creates better coupling, but also tends to decrease frequency
flatness. The cool thing is that all ports drive 50 ohms, and the
pick-off's output is surprisingly flat over more than an octave.

Any ideas? It's definitely not constructed the way I'd build a directional
coupler -- even though it performs a somewhat similar function --, nor a
"magic tee" (although I've only seem magic tees in the form of waveguides
and transformer-based affairs for HF).

Thanks,
---Joel Kolstad