Well, here's some favorite late adds to this thread:

There once was a poet named McNamiter,
Whose tool was of prodigious diameter,
But it wasn't the size,
Gave the gals the surprise,
Twas his rhythm – iambic pentameter!

There was a young widow named Brice,
Who kept her dead husband on ice;
She said,"Twas hard when I lost him,
I'll never defrost him,
It's rather cold comfort but nice."

And for the nautical crowd:

There once was a pirate named Gates,
Who thought he'd do battle on skates,
But he fell on his cutlass,
Which rendered him nutless,
And practically useless on dates.

Arghhh!