I love my DIY bike trailer! (Piwoslaw linked to it, above.)
I've done the kayak thing with this one as well.
Just yesterday I was thinking about making another bike trailer that would let me pull a ~140 lbs catamaran down to the river, instead of driving it down:
If I take one hull off, lash it to the other, tip everything to stand upright on its side, it would only be about 2.5 ft wide, with most of the mass down low - appropriate for pulling behind a bike.
Probably won't do it this year though - too many projects.