Magic: The Gathering Arena is starting to gain some momentum and if the developers can solve the dreaded "fifth card problem" (read: rewarding players fairly for getting cards they don't need) and shape up the economy it has a long run ahead of it and the potential to shake up the entire digital card game market. Right now it's in open beta form following a closed beta reset, which Wizards of the Coast calls "the last planned reset."
So you can spend real money but it still very much is a beta, lacking several major features like...playing with friends. That changes on November 15 though, as the "direct challenge" system will be patched into the game at that time, allowing folks to directly meet up and play for fun. No quests can be completed in direct challenge at this time so don't get too excited.
At this point, even with the reset, I've built up a formidable collection thanks to smart drafting. Right now I'm running a Golgari Midrange deck, as well as Izzet Control, Elfball, Mono Red and my old classic: Vampires. Next I'm going to bolster my Merfolk deck and possibly create a Blue Tempo (I already own both in paper form).
Magic Arena [Facebook]