In my opinion, Jack Vance's Dying Earth is under-appreciated when it comes to inspiration for roleplaying games. While many know that Vance was the inspiration Gary's magic system in D&D, I find it to be one of the least useful aspects of the Dying Earth to replicate.
What's truly brilliant about Vance is his wilderness adventures. How one moment your trekking through the wilderness and the next you've found yourself in a nice little village that turn out to be unbelievably creepy or insane about halfway through the story. It's just awesome.
Not only that, but he has the most scheming cast of character I've ever seen in a fantasy novel. There's no Frodo going out to same the world from utter destruction. No. Instead, you've got Cudgel, the greedy womanizing bastard who somehow always seems to stay a step ahead of adversity. Just about every character knows what they want and will do just about any to reap the potential benefits. Actually, most of Vance's character act quite like the PCs in most games I've ran or played in.
Now, that doesn't mean I don't sometimes think Vance is a bit of a sadist on paper. He is a little bit, but at least he's a great one. No really, I bet you could pick up a the Dying Earth, flip to a random page and come up with a great villain for that night's session; I've done it.
So, that turned into more of a rant than I originally planned. Guess I have to include a new rule, monster, magic item, NPC, or something now:
Worms Upon the Lier
This spell must be cast one a creature making an oath. Any time thereafter that the oath is broken, the oath-breaker will erupt in the horde of writhing blue worms. These worms will fuse into two monstrous strands, eating away at oath-breaker's body. If the oath was not to divulge information of a curtain kind, the wormy tendrils (4HD each) will seek out the confident or interrogator and mash him into a blood-bony soup.