Spectromancer is an online "card" game. (The version I've been playing is subtitled "Gathering of Power.")
I enjoy this sort of thing, and Spectromancer is a pretty fun one.
Basically, each turn you either cast a spell or play a creature, and then each of your creatures that you played on previous turns attacks. Creatures that don't have an opposing creature to deal with in their own column attack directly, while others attack their opposing creatures. Simple enough.
Each creature tends to have a special ability or two. Each player also has four different powers to choose from plus an extra one determined by "class."
This makes for a pretty fair amount of variety for a flash game of this sort, and my only complaint is that you can't mix and match the "class" powers-though admittedly, many of those would be way too breakable if you could.
I don't know that I feel the game is tremendously balanced, but it's balanced pretty well, and the game's story was apparently contributed to by Richard Garfield, creator of Magic: The Gathering, the granddaddy of all card games of this general sort. (Though I haven't looked at the story thing, and it should be noted that Richard Garfield is actually a college professor with a Ph.D. in mathematics. Yes, really.)
Anyway, it's fun, and if you enjoy this sort of thing, you should check it out.