Hex

Hex

Designers: John Nash, Piet Hein (designed independently)
Type: Abstract
Status: Free/Public Domain
BGG entry: http://www.boardgamegeek.com/game/4112
Available at: http://www.cox-tv.com/games/rules/hex.html

From the BGG entry:

This is the abstract game John 'Beautiful Mind' Nash invented independently, along with Piet Hein (six years earlier). An 11x11 board was sold back in the 1960s. The game is almost certainly public domain now. It has the simplest ruleset of almost any game: connect your color edges with a path of tokens of your color. There are several resources for Hex on the Internet, including at least four realtime Hex servers, the most popular of which is now Kurnik. All these servers have the 'swap rule' implemented, which makes the game much more fair. You may be able to find a used set (or a very nice homemade set) for auction on eBay, but you can also buy a set piecemeal. A Battlemat from Chessex.com with 1 inch hexes can be cut into a 15x15 rhombus grid. Standard Go stones would work fine for this board.

Randy Cox has an excellent Hex page, complete with rules and printable boards:
http://www.cox-tv.com/games/rules/hex.html
A printable board is available from Hex's BGG entry:
http://www.boardgamegeek.com/game/4112