VCRatings have a writeup on a panel at O'Reilly's Web2.0 Expo where a panel on virtual worlds discussed 3 projects you may not have heard of before:
Club Penguin is aimed at six to ten year olds and is billed as a safe, fun and educational hangout for preteens. It appears to be a 2D browser based affair with an emphasis on making parents feel secure. The company is canadian, and apparently pretty popular.
Gaia Online is aimed at an older set. Aimed at teens, it already boasts 2M unique visitors a month, with an average visit time of 1hr and claims to host the second larget forum on the net. I've not tried to join/test but it again appears to be a browser based 2D world.
Areae is under development, but funded. From the report on VCRatings, it appears founder Ralph Koster is a bit of a nutter, but it takes all sorts...
Kids and teens worlds are HOT right now, with Mattel's Barbiegirls due to launch later this month, expect this space to get even more attention.