The Planeshift set featured a cycle of rare global enchantments with "Planeswalker" in their name. They're capable of generating repeating effects based on the contents of another player's (another planeswalker's) hand. Each card in the cycle featured a different planeswalker from the Magic storyline in its art.
White: Planeswalker's Mirth
The white enchantment in the cycle shows Commodore Guff chuckling his way through a lifegain effect.
Blue: Planeswalker's Mischief
The blue one shows Bo Levar firing off a mischievous blue effect.
Black: Planeswalker's Scorn
You can see Tevesh Szat performing some anti-personnel necromantic magic on the black one.
Red: Planeswalker's Fury
The red one shows the planeswalker Lord Windgrace doing fiery battle with a Phyrexian minion.
Green: Planeswalker's Favor
Finally, the green member of the cycle shows Freyalise conjuring some creature-enhancing magic.