This is my favorite. It is a jigsaw puzzle that is new everyday. You can change the type of cut and number of pieces.
Were you a fan of the old Simon game? You can play it online.
How about Tantrix? Turn the tiles to make a shape of the right color.
Make your own painting at this site. The faster you move your mouse the smaller the paint lines will be. Every time you left click the color changes.
That's all. You can't stay on the computer all day! I found these on Amys's blog. If you want to see more, check out her blog at Experience Imagination.