I call it Mini Spectrum, but I'm sure somebody else could come up with a better name for it. It labors under the following restrictions. Your deck must consist of:
Plains x3
Island x3
Swamp x3
Mountain x3
Forest x3
Cards of your choice x15
All cards are restricted to one copy per deck (except I guess Relentless Rats?).
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It's basically just a constructed version of Mini Masters. Like Commander, games tend to be long(ish) and unfocused. The deck building restrictions make card choice very interesting since you can't skimp too much on any one color (although an artifact deck could probably do fairly well), and need to watch out for cards that require more than one colored mana.
I've played quite a bit of it with my wife, and while we haven't worked really hard to build busted decks or anything, I think it's definitely got potential to be a fun format. If anybody tries it out, let me know in the comments, I'd certainly enjoy hearing experiences besides my own.