Searching by date of publication of a web page is my all-time favorite Google feature.
My biggest grouse about the Internet is that many academic and technical articles don't carry a date of publication. As some topics lose relevance over a period of time, it is hard to judge if the piece you just finished reading is still useful. Typically, all search engines get you the most popular links for what you're searching in precedence to new and more accurate material.
The Any Time drop down menu visible when you click on the Search Tools button on Google has been helpful to me in my research activities a lot of times by getting me the most recent and accurate results on time-sensitive topics.
Also see:
My favorite Chrome Extensions
My favorite jQuery plugins
My favorite Foxit Reader keyboard shortcuts