A wonderful new tool I have discovered recently is Behold, which is a search engine for images.
Of course, you can search images with a number of different search engines. The things that makes Behold different are that:
1. Behold searches among Flikr photos, particularly those that can be used freely (for non-commerical purposes) under a Creative Commons license (that is, people who are willing to share their photos for non-money-making purposes without the usual copyright restrictions).
2. Behold somehow filters the photos, and searches among only the million plus photos it considers to be "high quality."
3. Behold has a way of combining image search tags, so that after you bring up "buildings," you can search for "towers" or "skylines" or other subsets of that tag.
I don't know exactly how it does what it does, but I will say that every search I've done with Behold has come up with professionally-quality photographs, not just any old thing that anybody has taken with a cheap disposal camera.
More importantly, Behold is a source we can use with our students to help them appreciate intellectual property rights. All too often, students feels like they can just take any image they want from the internet and use it in their own work. So it is great to direct them to a search engine like Behold where they can restrict their searches to photos where people have agreed to share their creations (under the perimeters of the Creative Commons license).