Kongai is the flagship flash game for Kongregate, a free online flash game aggregating site. In it, you earn collectible cards, then assemble them into "decks" of three or five cards, then venture out and do battle with other online users.
The system seems complex at first, but the battle system is basically a souped-up version of rock-paper-scissors. All the cards are equally good -- not equal, per se, as each has different strengths and weaknesses, but there's now one set of cards that's guaranteed to dominate all other decks. In addition to the fighters, you also collect items, which enhance the characters in certain ways to cover up for their weaknesses or double up their strengths.
The game is extremely well balanced, and successful players learn to read their opponent's minds. The most fun aspect of the game is the tag team action, in which you swap one card for another mid-battle. If you opponent guesses correctly you're about to switch out, they intercept your character and do a great deal of damage -- but if they guess wrong, they just wasted a turn.
Balanced play (no inherent advantages to the cards)
Simple play structure with complex strategies
Stable multiplayer interface with rankings
Incentives to win in the form of new cards
To build up a good variety of cards, you must play other games at Kongregate (which is the point of the game for them)
You don't really select your opponent, but get matched up with a random player of about your skill level or ranking
It takes a long time to get new cards, so pick those starters carefully
You have to pick starter cards before playing, when you don't really have a feel for what does what
The artwork is misogynistic
You need a fairly robust internet connection
Overall, Kongai does what the best free flash games should do: it provides hours of addicting and engaging multiplayer action without the need for an expensive console, and each game is short enough to fit into a quick work break. ;-)