Here is a picture:

{{gwi:598005}}

looks like a coprinus species...i.e. "inky cap" because the cap "dissolves" in a black gooey mess after a day or two.

I believe you're right. They look like a small version of this:

{{gwi:598007}}

This is Coprinus comatus if anyone is interested. I'm still confused though how it popped up even after every living thing should have been fried in the microwave though.

OK--so these keep coming back like every day. What is the best way to get rid of them? I doubt they're beneficial to my plants...

They probably are beneficial, actually. They're saprophytes and they break down nutrients in the soil and make it accessible to plants. They're certainly not competing with the plants for food. If they're parasitic (and most coprinus species, from what I know, are not parasitic at all), you'll see pretty immediate sickness of your plants. I would just leave them. Nature has created these types of associations over millions of years of trial and error.

Ah, Midnight Mushrooms....!
I agree with Stiffy, no harm from the mushroom. They'll only last
seasonally anyhow.

Josh