Back in the 20th century, they were known as colloids, but nowadays, if you suspend nanoparticles in a fluid, the result has to be a nanofluid. The rebranding also served to advertise the effect that the thermal conductivity of some colloids appears to be greatly enhanced for reasons not entirely understood so far.
Cooling applications where minimal size and weight are an issue (cooling computers, aircraft) will surely benefit. I'm rather sceptical about heating applications and any context where the size of the radiator doesn't really matter.
I've done a feature article on this which is out in Chemistry&Industry issue 16 (27.8.), p. 19.
It should turn up on their website, but as of today it hasn't been updated yet.