Recently I stumbled upon a gem of an online CCG,
HD Xyth.
The gameplay is pretty nice - it's quite similar to the free CCG Elements, actually, though its sci-fi themed. Like Elements, the only restriction on the cards you can play is the amount of energy you have. Like quanta in Elements, there are different types of energy that you get from deploying a limited number of structures or some actions. Each in-game race uses one type of energy, though some cards are dual-classed. Because you can only have five structures, and there are some that offer utility besides generating energy, you actually need to think about how many structures you put into your deck. Note that some actually cost energy to build, mostly ones that do other things besides generate energy. With energy you can build various units, activate actions, or use activated abilities on units. Like with structures, you're limited in number, though its seven as opposed to five. In this respect its very different from Elements, because in that game you're probably not going to worry about space nearly as much as you would in HD Xyth.
There seem to be a variety of game modes: it has both multiplayer, and singleplayer. Haven't played much multiplayer because there doesn't seem to be anyone around, but there are a couple of different modes in single player. There are game campaigns (one for each race), single duels, some kind of war between races thing (haven't really played this yet), and a daily challenge. Best of all, unlike every online CCG ever, it doesn't demand that you pay real cash to get anywhere (it's literally impossible to pay for in-game advantages, actually) and unlike Elements it doesn't need much grinding, or any grinding at all really. You seem to start with a ton of cards for every race and can get them easily enough just by playing, because you get some randomly after winning and the campaign gives plenty for the race that you play, and you can choose a "favorite race" early on that gives you cards for one race. You also get cards for achievements, which for the most part are very reasonable. There's a laboratory thing where you can take duplicates and upgrade them (mostly just small bonuses and additions in utility, nothing big), combine cards from different races (haven't really experimented with this much yet), or turn them into tokens. There's also a card-trader thing where you take the tokens or certain cards from your collection and trade them for what the trader offers, but there's a limited number of offers per day. There's also a slot machine thing that can use tokens; you also get a free spin per day, so there's something.
I'd love to see some other B12ers on this game, it's actually really cool and a lot of fun to play once you've got a decent grasp on it. It would be great to play some of you guys in multiplayer, actually. If you enjoy online card games and hate annoying grinding/pay-to-win elements like me, it's pretty amazing.