SAN SERO Vented Microwave Cover. I was just thinking about this the other day. I've owned this thing for over 9 years and use it nearly every day. At the time I think I paid like $20 for it (it's $34 now). And I thought that might be silly for what is basically a piece of plastic.
But holy crap, it's got to be my highest value-to-dollar ratio item EVER.
With it, I hardly ever have to clean my microwave. It keeps splatters to the cover itself, which is trivial to wash out.
And it helps with energy efficiency too, keeping heat close to the food, instead of it being lost as waste heat.
It comes with tiny vent holes at the top, so it won't steam up much or pop off.
And it has a nice handle built into the top middle of it, so most of the time you can grab that without burning yourself.
It also fits perfectly onto your typical round dinner plate. I mean perfectly.
This thing has been with me through four different homes/house moves, and hundreds of times of cooking or re-heating items on bowls and plates.
It's easily the best thing I've bought in my life.