If you search for 'atari breakout' and then click on Images, you get a Google-ified version of the old-school game. You have to bounce the ball off your paddle (moved with the mouse) to destroy all the tiles!
if you search for 'zerg rush', you get a secret game where letter Os fall down and start obliterating search results! Try and click them away before everything you've searched for is destroyed!
Searching for 'pac-man' gets you a miniature version of the classic game (as beloved by Guardians Of The Galaxy's Star-Lord), where Pac-Man has to avoid ghosts around a retro version of the Google logo.
The Hidden T-Rex Game!
If you use Chrome to browse the web, if you turn your Wi-Fi off you'll reveal a secret (and incredibly addictive) game where you jump a T-Rex over a bunch of cacti!
If you search for 'do a barrel roll', your browser window responds by... doing a barrel roll! Also, if you search for 'askew', your results come back all tilted!