Take Halloween pictures from the past years, as far back as you want, and print out 2 - 2x2 squares of each picture. Mount them on card stock, Halloween style if you want, and cut those out into squares. You should have 2 of every picture. That's it!
Place the squares picture side down, in nice even rows, and take turns picking 2 cards at a time while trying to make a match. Whoever has the most pairs at the end of the game, WINS! My little ones LOVE this game. I'm putting together my family's Halloween Memory game today and I'm sure we'll get lots of use out of this easy, cheap, homemade game!