This article from Smashing Magazine is so inspiring. It lists some of the pioneers of the collage technique, then lists some great modern works. At the end there are links to more information including tutorials on working with mixed media and collage as well as some museums and organizations that focus on this form.
This is great stuff to spark your creativity. Whether you are doing altered art, bookmaking and journaling, scrapbooking, web design, or just looking at these striking art forms, this is a great visual essay.