Although I am not a fan of Mensa, their kids' site is really good. Has stuff for lots of different age groups, and my kids love it. I don't remember the URL, but google "Mensa for kids" and it's right at the top.
Yep, that's pretty much the website address in and of itself!
http://www.mensaforkids.org/Out of curiosity, why do you say that you're not a fan of Mensa? I've enjoyed some of the members, but I do have to say that I am a bit bothered by some of the commercialism/push for all family members to join in order to access events that I am seeing in the GCC discussions as of late. The organizational policies that are aggravating me and the members are different entities in my mind, though, lol!
My girls are not really into online educational stuff anymore unless they are looking stuff up for school (they're 11 and 13). However a few sites that come to mind, depending on what you're looking for (videos vs. educational game type of sites like starfall) might be:
http://www.watchknowlearn.org/default.aspxhttp://pbskids.org/cyberchase/ (my kids loved this PBS show when they were younger and the website has videos, math games, etc.)
http://freerice.com/#/english-vocabulary/1399 (vocab, probably geared more toward late elementary for gifties than very young kids unless they are very adept readers, although I just now saw some really, really easy words come up, so maybe good for younger kids too)
http://www.edheads.org/# (under the "choose an activity" tab on the top right, you can select virtual surgeries to perform)
