Freecell for Mac OS X—download here.
Freecell is a solitaire card game in which almost any deal can be won, if you have enough skill. The card table consists of four free cells (top left), four stacks (top right), and eight columns (bottom).
A shuffled deck of cards is dealt face up into eight columns. Your goal is to move all of the cards from the columns to the stacks. The stacks must be made in rank order, starting with aces, then twos, threes, and so on.
Cards may be moved between columns, within some rules. You can only move one card onto another which has the next rank up, and the cards must be of opposite colour. For example, you can move the five of hearts onto the six of clubs.
Finally, cards may be moved to and from the free cells at the top left. Each free cell can hold one card at a time. Empty columns act like free cells, too.