Deirdre and I have both enjoyed Puzzle Pirates immensely, and the developer Three Rings recently launched a new title, Spiral Knights.
This game is much more mainstream. It's like a Diablo-Zelda mashup of some sort, wherein players delve down into dungeons, earn loot, and hack at enemies with swords. Its gameplay is entirely party-based, with a good matchmaking system. Dungeons are instances generated for your party, so the only time you see more than 3 other people is in the town hub, where I guess people labor communally to unlock more dungeons.
It borrows a gameplay limiter in the form of Energy, which you use like a currency to access dungeons. Each floor down costs a little more Energy, which replenishes up to 100 per day. Other things cost energy too, like crafting. This is all well and good, because Energy is the game's form of premium currency.
Basically it works like this: you can buy Energy crystals for real dollars, and either use those to play for longer periods per day, or you can exchange them in in-game markets for in-game currency. Three Rings used a very similar mechanic in Puzzle Pirates, where the freemium oceans were free to play, but just about every economic transaction required Doubloons, which players could buy for real money. The Doubloon Exchange, however, allowed players to trade Doubloons for the in-game currency, Pieces of Eight (poe).
This is a business strategy I wholeheartedly support. For one thing, it makes the game accessible. If you haven't got money, you don't have to pay money. But the exchange is key: it makes it so there is a definite "class" advantage for those willing to pay, but nothing is inaccessible to players playing for free. If you're patient, you can earn in-game wealth enough to trade for the premium currency. On the other hand, if you're short on time, you can pay real dollars to sell in the market to advance wealth faster.
Interesting, the incentive structure for Puzzle Pirates made it so that free players had to play more to accumulate wealth. In Spiral Knights, the depletion of Energy puts a hard cap on how long you can play, and consequently how much wealth you can accumulate to trade for more Energy. Buying Energy to play longer using in-game money, in theory, should be a net gain for the buyer; if you can't use that Energy to make back the wealth you spent getting it, what is the point? What price should a player be willing to pay for extra Energy in order to break even?
