Microwave....but watch closely and only zap it for 2 to 3s econds at a time. you don't want to melt it.
Or use my trick...get the butter out of the refrig the day before...it really won't go bad in 24 hours.
Linda C

Depending on 'how fast' you need it - let hot water from the faucet run through a bowl in your sink then take the empty, but quite warm, bowl and place it upside down on a stick of butter (that has been cut into quarters) on the counter. It will soften up in a bit.

I've also put my toaster oven on for a minute or two and then put a stick of butter (cut up in pieces) inside just to soften up.

Cut into pieces. It softens faster.

You can soften it in the microwave - unwrap it and put it on a plate and microwave for 1 minute at 10% power. Ginger is right, cut it into pieces.

I cut a stick into 1/8ths then put them in a bowl under the warming light of the Vent-a-Hood. 8 mins. and it's "room temperature."

I put the whole stick in a zip lock baggie and set in a bowl of warm water.