When I have half an hour of free time, I sometimes use this to play
Hearthstone, that free Warcraft themed card game by Blizzard. It is quite popular,
having just reached 50 million players recently. The game was designed so that you either need to play a lot of games with characters or techniques you actually don't want to in order to earn virtual currency, in order to craft or buy more powerful cards you need in order to win against others. Or to simply buy the cards directly, using real world money. But it is still interesting to play anyway.

I didn't spend money on the game, but have a very nice set of powerful cards already, and recently was even able to create a really nice deck, which surprisingly worked very well: I made it with my new deck within about 20 or 30 games up to level 8. Which is very much for me: As casual player, I'm usually hanging around level 19 or similar. I'm not simply creating decks by looking into the web and copying a successful deck as most people do as it seems. I'm more having fun with experimenting and trying out some own ideas.
Unfortunately, Hearthstone updated a few days ago, and introduced several
Nerfs, making 2 of the cards which my deck relies on much less powerful. Rendering my deck worthless. Which is quite a bit pity. I was quite happy after that much time casually playing, and finally having found a deck that worked. With this nerf, now Blizzard has effectively destroyed the fun I had with this game.
I thought a bit about this, and came to the conclusion that it would probably be better to quit playing Heathstone. It is actually not fun playing, it's just making you a bit addicted. And once I recently found a way where the game was actually fun, they immediately destroyed that again. I think games should be fun, and not addicting. Of course, it is more difficult to make money from this that way.
Getting a community together to actually make the open source alternatives turns out to be more challenging than one might like to imagine, eh?