I was reading a question here on Food and Cooking and no one seemed to know how much it is. I tried to Google convert it to cups but no dice.
So how much is it?
In this Gordon Ramsey scrambled eggs video he uses a "knob" of butter. It appears to be about 2 Tbsp.
I don't think it's intended to be a specific term. You'd never see "knob" used for baking, where exact amounts matter. When cooking, recipes tend to be a general guideline rather than a strict set of instructions.
I tend to treat it as 'a knifeful' but thinking about now it its probably about 1-1.5 tbsp, depending on how soft the butter is.
If the butter is too cold to get a knife to scoop it with, and I have to cut it, I try to cut a corner off starting about 1 cm into the edge, creating a pyramid type shape.
As @hobodave pointed out Gordon Ramsey uses about 2 tbsp, but that tends to be the same with the chefs, they go heavy on the salt and the butter and the cream.
I believe the term "knob" of butter was developed and used in the days before standardized measuring. The amount is about 1 1/2 - 2 tablespoons -- about the size of a knob on a kitchen drawer.
Other non-exact measurements: palm, handful, wooden spoonful, size of a walnut, size of an egg, a glassful ...