A few of my favorites (as much because they're fun to say as diffecult to solve):

Marble walls as white as milk,
and skin covering all as soft as silk.
In a fountain, crystal clear, a
a golden apple doth appear.
No doors are there to this stronghold
Yet thieves broke in and stole the gold.
---
Old mother Snitchet-Snatchet had but one eye
And a long tail, which she let fly
And every time she leapt over the gap,
She left a bit of her tail in the trap.
---
I went to the woods and caught it.
I sat me down and sought it.
Because I could not find it,
Home with me I brought it.