I'm a huge fan of the free tools that Google offers, as they integrate perfectly into my classroom. Google's best offering, though, is its original--search. Even with some recent competition hot on its heels, Google Search is the best search on the web. Here are four modifiers that I use on a regular basis that make Google Search even more useful.
Using site: followed directly by a website URL or a domain type (no space) will give you results just for that specific page or domain type. For example: You want to find resources for online safety on pbskids.org. Simply search for "online safety site:pbskids.org" and you'll only get results for those words on the pbskids.org website. Or say a student is researching Shakespeare and wants to use a .edu website as a source. Have him/her search for "Shakespeare site:edu" and it will return the results for Shakespeare on .edu websites.
The filetype: modifier allows you to search for files of a particular type, such as PowerPoint (ppt), Word document (doc), PDF, etc. Say you want to find a timeline of World War II, but you want it in a Word document so you can edit it and print it for your students. Search for "World War 2 timeline filetype:doc" and you'll only get Word documents in your results.
Ever been on a website and wished there were an easy way to find other websites like it? The related: modifier is it. Imagine you're browsing the Edutopia website, and you decide that you want to find similar websites. Start a Google search for "related:edutopia.org." Google will spit out a list of websites that are similar in content to Edutopia!
Here's the situation: You're perusing your Google Reader RSS feeds, reading an article about recent viruses, and you run across the term "cybersquatting." Pretend for a moment that you have no idea what that means. Simply open up another tab and do a quick Google search for "define:cybersquatting." Google will give you several popular definitions from a variety of websites.
There are many more modifiers (like forecast:zip code for your local forecast), but these are the four that I've found most useful in an educational setting. They're great for me to use during prep and equally great for students to use during research. For more modifiers and other search tips, check out my "Better Google Search" presentation on my website, http://www.teachthecloud.com. If there are others that you've found useful, please add them in the comments!