Mostly my D&D-specific inspiration comes from sources outside RPG books. (Some of these sources have inspired RPG books I've worked on, so there's kind of sort of a synergy there: my wife's current assassin character and her origin are very Changeling: The Lost-inspired, but does that count? Technically we both are familiar with the sources that inspired C:tL, so...)
Damnation City, for Vampire: The Requiem... that book made me look differently at how I handle urban games in all kinds of genres. Normally cities are something I have a hard time wrapping my head around, but this really reminded me of the strengths to be gained from making districts in a city characters in their own right.
I picked up a couple of Pathfinder volumes lately, and the description of the city of Westcrown in one of them was a favorite. Again, I liked the emphasis on landmarks and districts rather than specific alehouses and inns. I've seen some fun and interesting monsters in Pathfinder as well; I do like reading about monsters that feel like they could have come out of myth.
I quite like Ari Marmell's Dragon articles on devils and demon lords. I'm generally a sucker for RPG infernalism (infernalists being perfect for players to remorselessly kill the living crap out of and make the world a better place), and his treatments of fellows like Beleth and Geryon are very enjoyable.
4e's DMG2 exudes love for running games, which is very nice. The Monster Manuals usually provide inspiration one way or another, sometimes with flavor text but most often (since I write my own flavor) in stat blocks and unusual power sets that I gleefully pillage and reskin. I may never use a krenshar per se, but that stat block is easily tweaked into an interesting scarecrow.
