On one of my development machines, which has seen a bit of fiddling it must be admitted, I find that the normal currency symbol for the locale is not displayed (be it £, $ or €). Instead I am presented with the following symbol: ¤.

Now I know that I have seen it before, but where? And secondly, what is it?

Answers:

1. In Interface Builder this symbol appears as part part of the format specification for a currency style instance of NSNumberFormatter.
2. It is known as the currency sign. It is used to denote a currency when the symbol for a particular locale is unavailable. Wikipedia has more (or on a mac, select the symbol, display context menu and select "Look Up in Dictionary").

More generally Wikipedia has an overview of currency signs and a useful reference on linguistic issues concerning the Euro.