A few of my personal favorites

Iplay
Iplay - a fun, friendly, and free Internet Hangout

NASA
The official site for NASA. Lots of galleries

The Daguerreian Society
The Daguerreian Society

Free Games Online - Your online source for free games on the internet of all kinds!
Lots of Free Games

RadarSat 1
Really cool if you like geology

Virtual Cleeve Abbey, Somerset, England, stunningly recreated by Virtex in three worlds
Some day all websites will look like this! *

Virtual Cultural Center & Museum
My crowning achievement *

* - Requires Viscape SVR