BeFree - this site has many mind-blowing videos from the yo-yo team in Poland.

The Glass Lab - trick videos, x and y axis spin labs, and much more.

HyperFusion - yo-yo mods.

HyperYo - Tyler Boshears site, with videos from contests and the Kansas club.

Infinite Illusions Message Board - the most active yo-yo message board...good stuff.

Ken's World on a String - arguably the best beginner's yo-yo site, with many trick
                                         illustrations.

The Next Level - yo-yo mods, and maintenance tips.

PhalanX - Spencer Berry's site, with videos of all those crazy tricks.

Sector Y - the best yo-yo website. Period.

Strings Attached - trick videos, trick illustrations, and other stuff.

Team Cypher - videos of SAGE doing his tricks.

YoCentral - videos from the yo-yo kids down in Texas.

Yomania - yo-yo reviews (more than any other site), and a cool message board.

The Yonomicon - the website for Mark McBride's infamous yo-yo book.

The Yo-Yo Underground - mods, videos (lots of freehand), etc.