Connect Four (also known as Captain's Mistress, Four Up, Plot Four, Find Four, Fourplay, Four in a Row and Four in a Line) is a two-player connection game in which the players first choose a color and then take turns dropping colored discs from the top into a seven-column, six-row vertically suspended grid. The pieces fall straight down, occupying the next available space within the column. The objective of the game is to connect four of one's own discs of the same color next to each other vertically, horizontally, or diagonally before your opponent. Connect Four is a strongly solved game. (с) Wikipedia
In my implementation of Connect Four you can play against a simple bot who tries to calculate the best move one step further or vs a random bot. You can also play against your real friend.
Game consists of around 1000 lines of code and is written in Java.