Gomoku (五目) is a japanese board game played with Go pieces and board (black and white stones on a 19x19 line board). The stones are placed on the intersections of the board, with black playing first. The first player to get five stones in a row wins.
A moment's analysis will reveal that having a row of four of your stones, with blank spaces at each end, is a winning condition... no matter which end your opponent blocks, you will win by placing a stone at the other end.
The next step before that, obviously, is having an open row of three stones; this must be blocked, or it will develop into the open row of four.
The next step is to form two open rows of three, which share a common stone, which when placed, wins the game: your opponent can't block the formation of an open row of four stones.
Beyond the most rank beginner ability, making this type of double open run of three stones is, interestingly enough, invalid.
Careful mathematical analysis (reference, please) has shown that the first player to move, with perfect play, will always win.
Various rule variants exist, including ninuki-renju, which was published in the USA as Pente.