Nearly every fantasy setting, from the Ultima series’ Britannia to the Wheel of Time, makes use of mystic portals. The principal characters need to see the key events that drive the plot, even if those events are thousands of miles apart. I’ve made use of various types of portals myself. If the portals are interesting enough, they can even become the focus of the adventure.
In the first installment of Perilous Gateways over at Wizards, Roger E. Moore describes the ancient network of portals created by the elven kingdom of Illefarn. Each article in the series will bring “more detail about a network of portals linking various parts of Faerûn and beyond in the Forgotten Realms campaign.” Handled carefully, the series could add to the Realms; handled poorly, the portals idea could unbalance many a campaign, as could the use of any mystical gateways.