Active Worlds was the coolest thing I ever goofed around in, at least at the time. The COFMeta (metaverse) and Mars worlds were my favorite.
It was basically the 3D equivalent of Geocities. You could wander for hours, looking at the cool little houses and weird displays people had built. You could even do simple scripting with keyframes for animating objects by toggling their visibility. :)
It was so cool, and I even ended up buying a little AWLD stock. The market imploded of course, and the people running it apparently did some scummy things (see Shamus Young's accounting here: http://www.shamusyoung.com/twentysidedtale/?p=13720 ).
The only thing really missing was modeling; to solve this, you'd run across giant prefab yards, and then write down model names. Mars had a sort of boneyard for this purpose.
They also did kinda-cool tie-ins with other media, like a world for The 13th Floor and the X-Files movie. I miss that sense of "Oh man, this is the future!".