These are the Google Easter Eggs.
If you go on Google and type in Zerg Rush a bunch of Os will appear and the pages you can go on start falling. If you double click, the cursor will turn into a plus sign and you can kill the Os and post your score on Google+. If you leave the Os alone for too long they will form two Gs.
If you go on Google and type in Google Gravity and click I'm Feeling Lucky, You'll be brought back to the Google homepage, move your cursor and all the stuff will fall.