When you are blogging, you often need to accompany your blog posts with images:
OK. Now pictures of kittens are obviously not necessarily ideal for business blog posts, but use your imagination and you can find all kinds of things. In order to find pages containing content that is free to use (or modify, or use for commercial purposes), you simply perform your normal search and then select 'advanced search' next to the search button on your Google results page.
On the advanced page, you want to select from the Usage rights drop-down:
Google explains a bit more about this feature in their help text. As you would imagine, it is looking for creative commons licenses on the target websites. Note that Google is careful to say that they don't actually verify that all the content on the sites is covered by the license - this is up to you to check, but it's still a huge time saver.
Now, you can obviously buy images from somewhere like iStockphoto, but sometimes blog posts call for 'quick and dirty' illustration. And for this, creative commons licensed information is perfect.
(The picture above was found using the search: kitten photo.)