Google

Holiday Logos - Logos for holidays and events

Logos - Logos submitted by users

Some logos unfold over a week's time (or more). They're called Doodles: I, II, III, IV, V, Dilbert Doodle

Easter Feature - Help the bunny capture falling eggs

Valentine Feature - Play with an animated heart

Google Zeitgeist - Search patterns, trends, and surprises

Google Labs - Beta testing for new ideas

elgooG - You even have to type your query backwards (!) to use this Google Mirror

Googlefight - compare hit results for opposing words or phrases. For example Cream vs. Sugar or Bill Gates vs. Steve Jobs

Googlism - What does Google think about (fill in the blank)?

Google Glossary - Find definitions for words, phrases and acronyms

Hidden Google Tools - from SearchEngineWatch.com