myHIP.com has created a great version of Tic Tac Toe which is challenging, fun and makes you think outside the box.  It's three levels of tic tac toe where you're playing against the computer.  The three levels makes for many ways to win/lose so this game stays fun for a while. 

I lost a whole bunch at first but once I saw the ways to win I started beating the computer pretty consistently.  Have fun!