Mac Spider is a very good Spider Solitaire game for the Mac. The goal of the game is to form a full group of cars from King to Ace and move them to the foundation piles at the top of the screen. You start up with random cards. You can group cars of the same suit in descending order, and you can move groups of cards around if they are in the correct order. For example, if you manage to group a 5, 4, and 3, you can drag those around and put them on top of a 6, for example. You do this all over the screen until you have a full group of cards. This tends to sound like its no big deal and anyone can do it. But not all the cards that you need are available every time, and after you run out of moves, you have to ask for new cards, which often mess up your game.
The difficulty of the game is established by how many suits you are playing with. By default, you play with just one, and this is not as easy as it sounds either. I spend a good 15 minutes to finish a game in this difficulty. And I have an OK understanding of the game. If you play with 4 suits, the game becomes really challenging.
I know people who play this game thousands of times and win only 10 or 20 games.
In short, Mac Spider provides a great Solitaire experience on the Mac. It costs $19.95, but if you are a fan, you can't go wrong with this game.