Mytheria is a free flash game that is designed to simulate a TCG/CCG-a trading/collectible/constructed card game.
It's obviously based (very) heavily on Magic: The Gathering, but eschews the use of cards as resources, instead tracking "power" (essentially the same thing as Magic's mana) automatically, letting you either increase your power or draw a card every turn.
This lets you quickly play out your hand in exchange for keeping a strong power curve (this phrase makes excellent sense to a TCG player) and you rarely have to worry about not having what you need.
The five colors, red, white, blue, purple, and black, aren't entirely equivalent to their Magic counterparts-in fact, where blue in Magic is control-oriented, blue in Mytheria is mostly similar to Magic's "White Weenie" archetype. (Not to mention that Magic has green, not purple.)
A further distinction is that Mytheria has very few cards by comparison to the venerable Magic. This lack of variety keeps numerous replays from being as interesting, but one supposes that the programmers only had so much time on their hands.
(Edit: Something I intended to mention but forgot is that while Magic allows a lot of flexibility in building decks, in Mytheria you're pretty much stuck playing a single color due to card design and game programming disfavoring otherwise; there are a tiny handful of "multicolored" cards, but they're all but unplayable, as are multicolored decks generally. Further, there are two or so cards which, while they are designed to "fit" certain colors, they don't actually need to be played with those colors, and are pretty good cards in the appropriate strategies because they aren't limited by normal play considerations, because you aren't limited by power when playing them. This also limits deck variety and pushes everything towards being more similar. Not a problem if you're playing casually, but...)
In short, Mytheria is a fun little game if you're interested in TCGs, but don't expect more than a few hours of play out of it before it gets really repetitive.