Quoted from CLAMP IN WONDERLAND’s official website:

In Japan, CLAMP will release the DVD of the animation clip “CLAMP IN WONDERLAND 1&2 1989-2006 (PRECIOUS EDITION),” featuring characters from all CLAMP works. We’d like to commemorate this occasion with a CLAMP net tour project. You can get special wallpapers available only on this site by traveling to various CLAMP-related websites and collecting keywords. For hints, please go to the CREATIVE LOG. Please look for keyword banners like above. When you collect all 17 passwords from the websites and type them in the box below, you will be directed to Secret Room, where wallpapers are waiting! Let’s get on a travel!!

EDIT 1:
Complete password list