The button may have been named as a play on the Clint Eastwood line in the movie Dirty Harry.
"Do you feel lucky, punk? Well, do you?"
Ordinarily when you type in a key phrase in a Google search, you press the search button, (you can also just press return or enter on your keyboard) and Google returns a results page that shows multiple Web sites matching your search phrase. The I'm Feeling Lucky button skips the search results page and goes directly to the first ranked page for that search phrase.
If you type "white house" in the search box and press I'm Feeling Lucky, you'll go straight to www.whitehouse.gov. If you type "apple" into the search box and press I'm Feeling Lucky, you'll go directly to Apple Computer's Web site. Depending on your search query, the first result is usually the very best guess, so hitting the I'm Feeling Lucky button saves you a few extra seconds parsing through the list of search results.
I'm Feeling Lucky is handy if you're fairly confident that the first result in the search engine is going to be exactly the page you want to find, but not so handy if you know you're going to be looking at a lot of sites. Using the I'm Feeling Lucky button is also a common way for people to point out Google bombs. It adds an element of surprise to the joke, but it only works if the Google Bomb is the very first result.
Lucky Button Without a Search
In December 2009, pressing the I'm Feeling Lucky button without putting anything in the search box yielded a countdown ticker to New Year's day. Pressing the I'm Feelinng Lucky button in 2011 gives you an explanation for I'm Feeling Lucky search, but if you've enabled Google Instant, it leads you to the Google Doodles page, where alternative Google logos are displayed.
Lucky in Other Google Tools
Picasa has an I'm Feeling Lucky button, but in Picasa's case, the button automatically applies enhancement filters and adjustments to an image.
I'm Feeling Lucky and Google Instant
When you're using Google Instant, the I'm Feeling Lucky button disappears as soon as you start typing. You could turn off Google Instant through your Google Account preferences, but where's the fun in that?
You can still use the I'm Feeling Lucky function with Google Instant by hovering your cursor over a prediction. Instead of a button, you'll see an I'm Feeling Lucky link to the right of the prediction. If you're feeling extra efficient, you can also navigate by using your arrow key. Press the down arrow to scroll through suggestions, and then press the right arrow if you feel lucky.