€

Symbol for the Euro, the European Union "single" currency.

HTML named entity : €

HTML numeric entity : €

Windows keyboard shortcut : Alt Gr+4 or Alt+0128 on the numeric keypad

Mac keyboard shortcut : Shift-Option-2 (thanks Spuunbenda)

And according to User Friendly, the Quake quad damage symbol on its side.

Note that not every browser/OS is fully capable of displaying the quake-on-its-side Euro symbol properly in all fonts. You might need to make an update for that. This writeup is being written in a browser that shows the symbol correctly in the title bar but not in the sans serif font used for the node header, for instance.

Windows Update is supposed to solve this problem for the most common used fonts, but chances are you have many others which still lack the symbol. No perfect solution as of now.

Same goes for Linux -- some of their fonts have the Euro, some doesn't.

€ is the other way of displaying the euro symbol, according to the revised ISO 8859-1 character set.