I'd say I'm most inspired by nature. Nothing makes me feel better and more ready to take on my own little corner of the world than taking a nice quiet stroll away from civilization.
Back in California, my folks live about an hour's drive from Kings Canyon National Park in the high Sierra, and anytime I visit home I make time to head up to the park to enjoy the scenery and wildlife for a day or two. There's nothing like the smell of the forest and the sound of a mountain stream to help you relax and forget about work for a while.
It probably comes as no surprise that I work best while listening to various 'sounds of nature' albums (with or without quiet background music). They help me to relax and concentrate much better than pretty much anything else.