Weird!

The name suggests it's intended to hose effects like Thalia, Guardian of Thraben. But a number of mechanics work with this: kicker, buyback, multikicker etc trigger this. Flashback might or might not, depending on the flashback cost :) Amusingly, morph does, because the facedown spell has no mana cost and CMC 0 but you pay to cast it (IIRC).

I think plain old Fireball too?

Yep.

Isn't the converted mana cost of a fireball spell equal to the amount you spent while it's on the stack?

I think it's mana cost is, but it's CMC isn't.

Back when Fireball had the errata, it was. But now it has an "additional cost" (separate from the ). That applies when casting the spell, but the spell's mana cost and CMC both only consider the X, not any additional costs.

In fact, there's a ruling on the Fireball link stating that:

> The additional cost is paid when you pay the rest of Fireball's costs, even though that text appears second on the card (to make the card easier to read). The additional cost doesn't count toward Fireball's converted mana cost.
> If, for example, X is 5 and you choose three target creatures, Fireball has a total cost of (even though its mana cost is just ). If those creatures are all still legal targets as Fireball resolves, it deals 1 damage to each of them.

Doh! I forgot fireball had an additional cost in addition to X. (I always end up using it at one target.)

I like it when somethingi make inspires this much discussion.
It is intended to work against Thalia-like effects, but of course it also works with all of the things you've listed. I knew that it would when I made it and hoped that that wasn't too much.