I used Flickr for a while a few years ago and found it frustrating. It had a limit on how much I could upload and tried to make me upgrade and pay to have more storage. I switched to shutterfly, mainly because my sister uses it for everything. In exploring it today it looks like a lot has changed since I last tried to use it. Several friends have created groups so that wedding guests can easily upload and share wedding pictures. I love that idea.
The searchable creative commons sites are great. It would definitely be useful for students who are putting together presentations, or teachers creating virtual field trips. I love this picture at the Library of Congress.