My favourite planet-hunting site is Centauri Dreams.  From there comes a discussion of the smallest conventional planet yet discovered outside our solar system--a super-Earth or mini-Neptune only three times Earth's mass.  It's not in a conventional location, however:  this planet circles a brown dwarf, a "failed" star that doesn't shine.

What's even more amazing (to me) is that there's speculation that this planet could be habitable.  There's a couple of reasons for this:  its size could mean that it retains enough radioactives in its core to heat it; its atmosphere might retain enough hydrogen (which is a greenhouse gas) to keep the surface temperature above the freezing point of water.  Also, although it's three Earth masses, that doesn't necessarily translate to three gravities of weight; it depends on its radius (you'd weigh almost the same on Saturn as you do on Earth, despite the fact that Saturn masses 95 times more than the Earth).

I never considered super-earths when I was inventing livable planets for my novel Permanence.  In this case, two interesting possibilities would be an oceanic planet with a hydrogen atmosphere; or a mini-Neptune with a radius large enough that its local gravity is Earth-normal, and an atmosphere that, like Venus, hosts a layer where the air pressure and temperature are also Earth-normal.  Just for interest, you could also imagine that the brown dwarf's radiation field dissociates water molecules at the top of this atmosphere; the hydrogen escapes and the oxygen falls back (this happens on Europa, which is now thought to have a breathable [by fish] ocean).  Then, you could have an air-world with Earth-levels of gravity, air pressure, temperature, and oxygen content in the air.  The only downsides:  no ground to walk on and no sunlight--ever.  But that lets us imagine all sorts of air-pirate scenarios in gloomy, lightning-lit skies.

Isn't that just too cool?  And brown dwarfs are everywhere.  As I said in Permanence, with this discovery the number of potentially habitable planetary systems in the galaxy has multiplied, by as much as a factor of ten.  There could easily be one within a light year of Earth.