A few years ago, I bought a rhyming dictionary. It was shrink-wrapped. Imagine my disappointment when I got home and opened it to discover that it was nothing more than a bunch of lists of words which rhyme with each other. I had quite reasonably expected it to be an ordinary dictionary where all the definitions were written in rhyme. You get the idea:
floater (n) sl.
In the loo
It seems to me there's a huge gap in the market for a proper rhyming dictionary.
(Twenty bonus points for the best rhyming definition in the comments.)
Postscript: I see from the comments that the gap in the market has already been filled.