What portals allow instant transit across continents?

• Stormwind and Orgrimmar to Twilight Highlands and vice versa.
• Stormwind and Orgrimmar to the Temple of Earth in Deepholm and vice versa.
• Stormwind and Orgrimmar to Vashj'ir.
• Stormwind and Orgrimmar to Uldum.
• Stormwind and Orgrimmar to Mount Hyjal.
• Darnassus to Exodar and vice versa.
• Undercity to Silvermoon City and vice versa.
• Darnassus, Exodar, Stormwind, Ironforge, Orgrimmar, Undercity, Thunder Bluff, and Silvermoon City to the Blasted Lands.
• Temple of Earth in Deepholm to Therazane faction in Deepholm and vice versa.
• Shattrath to Caverns of Time (Tanaris). This is not a normal portal, you need to talk to the Keeper of Time faction member in the World's End Tavern and have a reputation of Revered or above. The reputation requirement may have been dropped.
• Shattrath to the Isle of Quel'Danis.
• Dalaran to Caverns of Time (Tanaris).
• Ungoro Crater (northern edge, only accessible by flying) to Sholazar Basin and vice versa.
• The Kirin Tor rings that can teleport you to Dalaran (also not a portal, costs about 6,000 gold but is class and profession agnostic).
• The Blessed Medallion of Karabor can teleport you to the Black Temple entrance in Shadowmoon Valley in the Outlands, but this may be infeasible to obtain given it requires a long quest chain and a successful Illidan kill.
• The Outlands side of the Dark Portal to Stormwind and Ogrimmar.

That's the extent I can think of off the top of my head now that they've removed the portals in Dalaran and Shattrath.

Some professions and classes have other portals/transport abilities but I won't list them since it sounds like you want the generic options.