I really love having definitions at my fingertips. As search engine get more and more integrated into the browser, I can quickly look up words for spelling or fun from a variety of sources.

Another neat little tool, and pretty useful. All you do is type in define: xxxx and Google spits out definitions pulled from the Web. Who knows if the sources can be trusted, who knows if Google has gotten permission from the sites involved to rip their content... but it's a cool tool none the less. You can even find definitions for your name, if you are so inclined.