One series that I'm particularly fond of is the Malazan Book of the Fallen, by Steven Erikson. The amazing scope, breadth of history, compelling characters and epic plots of the books really have to be read to be believed, but I think it's safe to say that, other than the Wheel of Time, another sort of series altogether, the Malazan Book of the Fallen is the most ambitious fantasy series ever written. Lord of the Rings was ground-breaking and fantastic, but Erikson is a much more sophisticated writer.
One of the things that I like the most is the way that the books incorporate characters of various different power levels. From the demigod-like ascendants, all the way down to the frequently deranged swamp-dwelling High Marshals of the Mott Irregulars (they are ALL High Marshals in the Mott Irregulars), pretty much every level of combat prowess or arcane power is represented.
I think that any DM who would like to work high-level NPC's into their game can learn a lot from Erikson's books. In many cases, these high-level characters are powerful forces whose actions effect the less-powerful like natural disasters or forces of nature. When titans clash, the wise get the heck out of Dodge.
High-level NPC's don't just have to be mentors, rulers or antagonists. They can also be inscrutable wanderers who occasionally devastate a continent for reasons only they know. Or warring heroes whose battle destroy towns or countries. It's possible to set all kinds of adventures around these kind of events. Rescues, attacks that take advantage of the overall chaos, or even serving and supporting one of these powers, with or without their knowledge.
So if you want some inspiration on creating a world where level 30 characters exist, then give the Malazan Books of the Fallen a read, and tell Kharsa Orlong I said hi. From a fair distance.