As the name suggests, you can play it for free. It is, as Lut puts it, a "freemium" game. You get access to some of the content for free, and some of the content is available only to subscribers, and some objects have to be purchased individually for a one-time fee if you want to get them at all. It's a hybrid of all other massively-multiplayer-online-game revenue models; the only element missing is paid in-game advertisements.
The hybrid revenue model is oddly appropriate, given the hybrid nature of the game itself. Most, if not all, of the other MMOs around center around one basic game or type of game. For example, EverQuest, Warhammer: Age of Reckoning, World of Warcraft, and City of Heroes are all essentially combat-based leveling games; there may be crafting attached but there's no real gameplay involved in it -- it's "gather the ingredients through combat or by wandering the landscape looking for X, then combine by clicking". PvE and PvP may play differently, but the game engine is the same. You could argue that "wandering the landscape looking for X" is a game separate from the combat engine, but if so, it's not much of a game. Puzzle Pirates arguably offers more variety, with a multitude of different puzzle games and even card games all accessible from the same game world, and the puzzles addressing different needs in the game world. But Puzzle Pirates has clearly picked its core competency as puzzle games: it's implemented parlor games like poker and hearts, but all Puzzle Pirate's original games are basically variations on a theme of "detect patterns in the pieces on a game board and manipulate the pieces on it to match those patterns in the fastest and/or most efficient manner".
Free Realms, on the other hand, contains not only different games, but different styles of games. So far, I've tried its kart racing, demolition derby, puzzle game for mining (and a similar one for gardening), collectible card game, tower defense game, Wario-style mini-games for cooking and smelting, mini-games for footracing and finding things, and a leveling-based combat game. There are other games I haven't tried: chess, checkers, the mail-sorting game, and what bradhicks described as the "perpetual Easter Egg hunt" of adventurers, where you wander the map looking for stuff (Brad Hicks loves this feature, which perhaps goes to show that not everyone agrees with my assessment that wander-the-map-looking-for-X is a lame game). And probably more that I haven't even seen yet. The mechanism for introducing you to the different games-within-the-game is to give you quests, just like WoW or WAR: NPCs with question marks over their heads will give you directions to talk to another NPC or to start one of the sub-games.
I hesitate to call all the sub-games "minigames", because some of them are a lot more mini than others. All of them have a certain relative simplicity to them, from what I've seen. The kart-racing game has a variety of race tracks to do laps on and power-ups that you can fire off selectively along the way; it's fun but it doesn't have the detail you'd expect from a modern full-fledged racing simulator. The combat game felt like a dumbed-down, clunky version of Warhammer or City of Heroes or any of a number of other combat-leveling-games. Click the target, click the attack buttons, try to position yourself not to agro more mobs, hope you don't run out of hit points before whatever you're fighting does.
In fact, the combat game -- which forms the core of nearly every other MMO -- is easily the weakest part of Free Realms. Maybe it's better if you play one of the subscription-only combat classes, but after running through three or four combat instances, I'd had enough and didn't try any more. It didn't help that leveling in the brawler class was painfully slow, particularly compared to leveling in most of the other classes.
But combat aside, the other sub-games have been reasonably entertaining. The biggest downside is that of all the subgames, only two are designed for groups: the kart games and the combat. The combat isn't fun. The kart games are fun, but when I tried them they weren't fully implemented yet so you couldn't level up in them (that was a couple of weeks ago; I guess it might've been patched in by now, but it seems unlikely). The only other player interactive sub-game is the trading card game: you can't play it in teams, but you can play it one-on-one against other players.
One thing that appeals to me about Free Realms is that the variety of different games makes it feel more like a "world" to me than a typical MMO. The sprawling virtual landscapes typical of MMOs don't do much to make the place feel real when there's basically only one thing you can do: kill mobs and take their loot. I don't know that I'll stick with it. It doesn't have the variety of puzzle games that Puzzle Pirates has, and while most of the non-puzzle games are entertaining for a little while, they don't have staying power for me. Finally, the lack of interesting group content really hurts it for my purposes: I use games as my primary form of socialization, and Free Realms manages to be, in some ways, even less social than Puzzle Pirates. As one example: the TCG is its own platform basically 'outside' the shell of the game. Your character will show as connected to Fre Realms when you're playing it, and they can send /whispers to you ... but you won't receive them, not even after you leave the card game.
I might yet get sucked in to buying cards for the trading card game anyway, though. Every Free Realms account gets a starter deck, and you can get more cards by either (a) buying them for real money or (b) winning them in card duels against quest NPCs. I went through the entire quest chain and beat all but two of the optional NPC decks, and have a total of, I don't know, maybe 140 cards. As any player of a CCG knows, 140 cards is not enough. There may or may not be non-card quests that also give out cards as rewards, but it looks like the natural way to get more cards from here is to ... eep ... buy them. I probably don't actually want to do that. Still, I've played several rounds of the TCG against terrycloth, and enjoyed it even though he's crushed me something like 7:1. I often get frustrated if I lose a lot, but for some reason it hasn't bothered me with this. Possibly because the game is quick to play, around 10 minutes per game, so there's always "the next game" to look forward to. Still, it's a collectible card game, and those have been quite expensive for me to play in the past.
Oh, one other noteworthy aspect:
The game is really cute. Really cute. Sugary-sweet, go-brush-your-teeth-when-you're-done-pla