The Power Nine are undoubtedly some of the best cards in Magic: The Gathering.

For the unenlightened;
The Moxen - Artifacts, all of them cost 0 mana. Taps for one mana, according to the kind of Mox it is. This should be fairly easy to figure out. Basically, free land, meaning you can pull out big fatty creatures or powerful spells before your opponent can do anything at all.
Black Lotus - Artifact. Costs 0, you tap it and sacrifice it for 3 of any color mana. This means you can pull out a Ball Lightning first turn, raping your opponent's health down to about 2/3rds of what it should be first turn. Most valuable M:tG card existing.
Timetwister - You and your opponent shuffle your hands into your respective libraries and draw a new hand of 7 cards. Best in combonation with Moxen, meaning you can whip out fatties, empty your hand, then pull out a new one, giving you a rather large advantage over your opponent.
Time Walk - Costs 1 generic and 1 blue mana. You take another turn. Do I even need to explain how overpowered this is? Your opponent has their creatures tapped, so you run over and break their face. Not quite dead? Play this and BREAK FACE AGAIN.
Ancestral Recall - Costs one blue mana. You or your opponent draw 3 cards. This is insane. For reference, in the newer sets, Inspiration lets only you draw 2 cards for 3 generic and one blue mana. This is one more card, for 3 less mana, that can be used to deck your opponent or make them discard early on in the game. Rapage.

Well, naturally, these cards are all some of the most wanted by pretty much all players of M:tG. They're very old, and the sets they were in are no longer produced, meaning they're quite limited edition. Being in the first sets means that there weren't as many players are there are now, meaning less cards bought, meaning low amounts of these. If you ever see people selling old M:tG cards in a garage sale, flip through the cards. If you see these, you've just made yourself a pretty fair amount of cash.