Arcademic Games

http://www.arcademicskillbuilders.com/

Some really fun single and multi-player games - mostly math.  The multi-player games can be made either private (only people knowing your password can join) or public (anyone on the site when you're hosting a game can join your game.  Also, you can play against the computer.

ABCya! Elementary educational games

http://www.abcya.com/index.htm

Pretty cool! Divided out in grade level. 

http://www.gamequarium.com

Gamequarium – A portal to hundreds of free, online learning games and activities for Pre-School through sixth grade (or so) (on a personal note, once I click past the homepage, I turn my volume off – the “Under the Sea” theme as a midi file drives me NUTS! LOL but my 6 and 9 year old nephew and niece really like this site ) 

http://www.mathsisfun.com/

Lots of math stuff – worksheets, information, games, etc. 

http://www.mathmastery.com/cyberchallenge/index.cfm

online challenge game - +, -, 8, and / done online in a game format! Pretty cool!

http://www.mathmastery.com/index.cfm

There are word problems that are pretty interesting at "DailyBrains Library" Lots of stuff at this site! Including the opportunity to enroll in their program. If your principal is a member of National Association of Elementary School Principals, there is an opportunity to get one month free. Something to take a look at, I think.