Solitaire! is a completely free version of the very popular Klondike game, which most people just call "solitaire".
Klondike provides convenient features such as smart-dragging, one-click moves, and autoplay that combine to reduce the amount of tedious movement during the play. Other features include the ability to use any image on your Mac as a card back or game background, unlimited undo/redo of moves, statistics, automatic game save/restore, and user-adjustable card spacing in the tableau.