There have been a couple of mood board generators before, but this new one called Niice is well, pretty nice. Just search for a keyword and Niice looks through its hand picked database for images that relate to your search terms. And it is chock full of design oriented images.
For example, here's a mood board for the word "Bold"
A free account lets you have three mood boards and allows for live editing, but the paid accounts really piles it on. The ability to export your board, drop box sync, image upload and even custom mood board styles.