Picasa is one of the most popular online photo publishing services available. However, it has always been a place to publish your photos for others to see. Now Google (the owners of Picasa) have introduced Collaborative Picasa Web Albums. Every album on Picasa Web Albums is potentially collaborative: multiple people can add pictures to the same album.
You can now create a Picasa account and then your, your siblings, your cousins, and others can upload pictures to the same albums. This should be a great method of sharing family photos, whether they be from the late 1800s or from last week's family reunion.