A game of tic tac toe with in built AI. With a little tweak you could randomize the AI's strategy so the game won't always end in a draw.

tictactoe.py

import random

# the board we print out
board = ['[0]','[1]','[2]','[3]','[4]','[5]','[6]','[7]','[8]']
# the list of winning combinations
win_list = [[0,1,2],[3,4,5],[6,7,8],[0,3,6],[1,4,7],[2,5,8],[0,4,8],[2,4,6]] 

X = '\033[92m' + ' X ' + '\033[0m' # makes the x green in *nix systems
O = '\033[91m' + ' O ' + '\033[0m' # makes the o red in *nix systems

# stylizes text

def style_text(string):
    print('#' * 20)            
    print(string)
    print('#' * 20)

# prints the current board layout

def current_board():
    print board[0],board[1],board[2]
    print board[3],board[4],board[5]
    print board[6],board[7],board[8] 

# checks the win list to see if 3 in a row has occurred
# if not, checks to see if a draw and if not does nothing 

def check_board(marker,player):
    for win in win_list:
        if board[win[0]] + board[win[1]] + board[win[2]] == marker * 3:
            style_text('Player %s wins' % player)   
            current_board()
            exit(0)
    if board.count(X) + board.count(O) == 9:
        style_text('Draw!')            
        current_board()
        exit(0)

def ai_turn(marker,player):
    hot_spots = [0,2,6,8] # this is the list of the corners to pick from
    combos = [1,3,5,7]
    random.shuffle(combos)
    random.shuffle(hot_spots) # we shuffle them so it's not predictable

    # First, attempt to win if it has 2 in a row anywhere
    
    for grid in win_list:
        if board[grid[0]].count(O) + board[grid[1]].count(O) + board[grid[2]].count(O) == 2:
            for win in grid:
                if board[win] != O and board[win] != X:
                    return win

    # Second, if it  can't win it will see if it can block a combo

    for grid in win_list:
        if board[grid[0]].count(X) + board[grid[1]].count(X) + board[grid[2]].count(X) == 2:
            for win in grid:
                if board[win] != O and board[win] != X:
                    return win    
                              
    # If there isn't a combo, see if you can block a dual set up
    
    if board[1] == X and board[3] == X and board[0] != X and board[0] != O:
        return 0
    if board[1] == X and board[5] == X and board[2] != X and board[2] != O:
        return 2
    if board[7] == X and board[3] == X and board[6] != X and board[6] != O:
        return 6
    if board[7] == X and board[5] == X and board[8] != X and board[8] != O:
        return 8            
                              
    # If it can't win or block, it will try to go for the middle
               
    if board[4] != X and board[4] != O:
        return 4

    # If it managed to get the middle, it'll combo via columns or rows                
    
    if board[4] == O:
        for spot in combos:
            if board[spot] != X and board[spot] != O:
                return spot 

    # If the middle is taken, it will go for one of the corners    
        
    for spot in hot_spots:
        if board[spot] != X and board[spot] != O:
            return spot                

    # Failing all other options, it will go for an available square at random
             
    while True:
        i = random.randrange(len(board))   
        if board[i] != X and board[i] != O:
            return i
                        
def main():
    global board        
    player = 'One'
    marker = X
    while True:
        if marker == X:
            style_text("Player %s's Turn" % player)
            print
            current_board()
            print 
            answer = int(input('Please select where to place your marker: '))
            print 
        else:
            answer = ai_turn(marker,player)
        
        if board[answer] == X or board[answer] == O:
            print('Please select a square that is not already used.')
            continue
        else:
            board[answer] = marker            
            check_board(marker,player)
    
        if marker == X:
            player = 'AI'
            marker = O
        else:
            player = 'One'
            marker = X    
main()                     

Updated the code so the markers are easier to spot, and added an extra piece of logic that closed a loophole, allowing the user to consistently win.