Wikipedia has all the basic species facts about monkeys, whether you are looking for information on old world or new world simians. There are photos of primates of all shapes and sizes, a myth busting section and an interesting article about monkeys in religion.
With "Fast Facts", such as diet, size, and weight, and a separate more detailed text, the National Geographic does a great job of exploring the Spider Monkey. The site also includes a sound clip of a real Spider Monkey and a video.
San Diego Zoo's official website has a page devoted to our simian friends, with lots of color photos and a thorough list of facts and information. There is a special section about how monkeys communicate with each other in their large groups.
What's the loudest animal in the world? The Black Howler Monkey! The National Zoo's Howler Monkey page is stuffed with interesting information, and even a link to "meet" the Zoo's two newest Black Howler Monkeys.
University of Michigan's Animal Diversity pages have a well-stocked profile on primates. This includes photos and color illustrations, sound bytes of howlers in action, and a photo gallery of various monkey skulls and bones.