Google is a puzzle for many of us, but until we find the last pieces, let's try Wei-Hwa's Puzzle Challenges, a module for Google Personalized Homepage with really tough puzzles. Perhaps because the puzzles are created (or at least carefully selected) by a Googler.
You can also participate to Google U.S. Puzzle Championship. The registration is open until June 15. Here's the test from 2005.
The brilliant Niniane Wang
Staying with the US Digital Service
35 minutes ago