Op-amps are one of the building blocks of the modern analog electronics industry.
Here’s an interesting one, the MAX4470 .. MAX4474 series:
Simple layout, again in a tiny SMD package:
The other members of this family are dual and quad versions, if you need more op-amps.
This chip is nice because of its phenominally low current consumption: 750 nA at 5V. It gets even better: at 3.3V, I measured a ridiculously low 190 nA!
Here are some more specs from the Maxim datasheet:
Might not be the highest-performance op-amp out there, but still – this thing could be quite handy to implement comparators, voltage followers, oscillators, amplifiers, filters, and more. Especially when the “power budget” is really really low.
PS. I’m assuming this chip isn’t oscillating with the above test setup, but in normal use you really need to tie the input pins to something to avoid that.