To many people stout is Guinness and that's that.   There are many others, but it is head & shoulders more famous than any other.

Stout was originally brewed all over the British Isles and exported abroad, where it was copied by the locals into their own versions.   The famous Dublin stout became the global brand it is now because of differing tax laws between England and Ireland in the 18th Century.   Unmalted barley was taxed at a lower rate in Ireland and was used in the production of Guinness stout, which gave it a competitive edge over English versions.   The resulting burnt, dry taste became the distinctive flavour of Irish stout as opposed to sweeter, maltier versions.

Supposedly the brand is now made with a percentage of beer brewed with Brettanomyces bacteria added to sour it.   Several websites mention this and several mention that it's unsubstantiated rumour.

Other stouts to try:

O'Hanlon's Port Stout - smooth and velvety.
Titanic Stout - good bitterness and burnt flavour.
Brooklyn Black Chocolate Stout - thick, chocolatey and warming.
Dark Star Espresso Stout - dry with aromatic coffee flavour.