Of the more obscure grounds (or no longer used) I've been to Folkstone, Bath, the old Hastings, Shenley, Radlett, and Tring.
All the usual ones like Lords, Oval etc, but the best have to be Tunbridge Wells, Arundel, and at number one Wormsley.
It was there I saw Essex play the Unicorns in April, and it was probably the coldest day I have ever watched cricket! Luckily there was a Yorkshire tea van, giving away free tea.