If you're about to start baking and have forgotten to take the butter out of the fridge ahead of time, all you need to soften it up is a rolling pin and a sheet of wax paper.
Unwrap the stick of butter and place it between two layers of wax paper. Use the rolling pin to beat the butter, then roll it out between the wax paper layers.
When the butter is spread flat, its larger surface area quickly brings it up to room temperature, and the wax paper's smooth, non-stick coating makes it easier to remove.
While this trick can work with plastic bags or parchment paper, wax paper is likely your best bet for quickly removing the butter from the inside due to its paraffin wax coating.
The rolling pin method is also better than using a microwave or double-boiler because there isn't excessive heat that could potentially melt the butter instead of softening it.