Yogile is a free, ridiculously simple photo-sharing service that lets you create public or private albums and let others easily contribute.
After a really quick sign-up, Yogile will let you create an album and upload content. Once you're all set, you'll have a URL for your album. If you made it public, all you have to do is send the link (which is customisable) to your friends and they'll be able to upload photos. If you made it private, just include the album password (not your account password!) during sign-up.
Uploading through the site isn't necessary, though. If you just want to have your group of friends upload photos via email, Yogile can handle that as well. If you want to give it a shot, send an email to [email protected] or visit my test album and upload a photo or two. At any point in the album's lifespan, you can download all the contributed photos in a zip file.