The TicTacToePro
"Tic Tac Toe Pro" is a 2 player iOS app, available in the Apple App Store.
You can choose from 3x3, 5x5, 7x7 or 9x9 boards.
3x3 is classic Tic Tac Toe.
5x5, 7x7, 9x9 all require 5 in a row to win.
There is also a challenge mode which gives you the opportunity to find the best position on a given board.