I've designed three full-sized sets of cards:
I've also designed three sets of "Bring Your Own Block" cards. You can read below about those and about how I got into this whole design-your-own-Magic-cards thing to begin with.
John is the guy who convinced me to participate in a "Build your Own Draft" event, which is just like a normal Magic draft, except that each player designs 45 cards beforehand. At the start of the draft, shuffle all the cards together, divide them into piles of 15, and those are the packs. (Details here.)
"Sure", I said, "that sounds like fun". So I fired up the old magic set editor and designed a set of cards. And then someone said that they would like to play but didn't have time to make their own set of cards. So I made another set of cards and even roped my friend Gavin Schnitzler into doing the art for four of them:
By then, I was getting pretty hooked on designing cards, so I made a third set and rather than just let it go waste, I found another Magic playing friend to pair it with.
The draft itself was a blast but looking back, many of the cards I made were poorly designed; some had too much text and most just weren't very interesting. There were some exceptions, though. I think Brainless Zombie, Rugged Ranger, and Ancient Mummy all had good flavor. Smart Lieutenant has a great combination of flavor text and art. And should I ever win the Magic Invitational, Wizard Numerologist will be my card proposal.
Having designed 3 * 45 = 135 cards, I was now ready for the next level: making an entire set. I immediately knew what I wanted the theme to be; as I describe in the design essays for the set, I thought that both flip cards and split cards had been poorly used by the current Magic designers. Thus was born Splitsville. The "pre-release" for the set was on April 25th, 2009, which is the day I had a bunch of people over to first play it. (I even wrote up a FAQ for them to read beforehand.)
Splitsville contained one reprint from my BYOD cards, the "five-way" split card, and one mechanic repeat: Wild Moment / Gone Wild re-used the "creature has no controller" idea from Gone Wild. My favorite cards from the set turned out to be the common cycle of move-with-a-flip creature enchantments (Magpie Wings / Magpie Beak, etc.) and the common cycle of half-the-time-their-dual lands ( Shifting Brush / Dried Up Brush, etc.); both cycles encouraged the fun mini-game of planning the flips so that the right ability was available when you wanted it.
And to this day, people who played Splitsville still talk about Engine Engine, though perhaps not in a 100% positive way.