Having switched EverQuest and EverQuest 2 over to the free-to-play model, SOE are trialling a new means of tempting gamers into putting down some real money. It's called the Krono and it's much like EVE's PLEX system in that it's a system that supposedly allows players to buy game time with in-game currency.
So, a Krono, once activated, gives you 30 days of game time and all the benefits that come with being a subscriber. Though coming by one isn't as easy as rooting about in the forest. Initially they can only be bought from the SOE store for $18 in real money, once a player buys a Krono it appears in their character's inventory and from there they can either use it themselves or trade it on the game's in-game market.
In theory this means that if you've very little in-game currency but a bit of real-world money going spare you could buy a Krono and trade it in-game for a bundle of virtual cash; or, conversely, if you're steeped in virtual money after 150 raids in a week you can go to the marketplace and hope that someone is trying to sell a Krono that you can bid on.
Currently Krono are only available in EverQuest and EverQuest 2, and once a Krono is put on the marketplace in either of those games then it is locked to that game, but the FAQ suggests we may start seeing Krono pop up in other SOE free-to-play games *cough* Planetside 2 *cough*
As I said, this is currently only a trial but if it takes off then it may become a permanent feature of SOE games.