Rupee
The small, glittering stones called Rupees are the currency of Hyrule. The inhabitants of Hyrule seem to be very sloppy with their economy, which has resulted in Rupees being found virtually everywhere throughout the land. Monsters that roam the land are also known to be carrying Rupees, which you can get a hold of by slaying the monster. Rupees are held in your wallet.
The colour of the Rupee indicates how much it is worth. The values are forever changing throughout the games but they do seem to loosely follow this pattern (exept Green blue and red which have stayed the same since a link to the past). Green Rupees are worth one Rupee; Blue ones are worth five; Yellow ones are worth 10;Red ones are worth 20; Purple are worth 50; and the Silver ones are worth a whopping 100! The most valuable Rupee however, is the Golden Rupee, which is worth 200.
Update: In Ocarina of Time there is a large Orange Rupee worth 500 and can be found at random by replaying the Diving Game at Zora's Domain and by killing a skull kid in the Lost Woods.
A purple rupee seen in the text of The Legend of Zelda: Twilight Princess as you find one. Worth 50, as stated above.