Here are some tools that can help spur your creativity. Don't try to execute every idea all at once -- pick a link at random and follow it.
The Brainstormer (Read a history of The Brainsormer here.)
The Psychic Sidekick
Directors Bureau Idea Generator
Michael Nobbs' 75 ways to Draw More and Draw Your Life
Doug Chayka's sketchbooks
A methodology for creating new ideas (written by professional illustrator Nate Williams)
An extensive list of ideation tools
Keith Haring knew that anything worth drawing once was worth drawing a hundred times.
I like to go to movies and draw in the dark. And I love love love blind gesture drawing.
Join the BookMooch Journal Project (or just browse their blog or their Flickr pool) or 1001 Journals
Participate in the quarterly Worldwide Sketch Crawl Day.
Illustration Friday suggests a new topic once a week!
Following are some idea-generation links oriented toward writers, but they could just as easily apply to image-makers.
No one cares what you had for lunch.
Idea Generator Blog Writing Prompts
Googobs of Creative Writing Prompts
Now rock out with your socks out.
How (and Why) to Title Your Work (Includes some prompts to help you create interesting titles.)
Project Idea: Object Sketchbook