I should of googled it first: shift + alt + 2 is € and alt + 3 is £

If you go to system preferences / input menu

And check "character palette",

You should end up with a little flag on the upper right of your screen. Click it to find a "show character palette" item and if you select that you can search all of the unicode characters and input them. You'll even have favorites you can add (there's tons more than you can type in on a normal keyboard)

You also have a keyboard viewer you can enable in the same way, it'll show where all of those you can enter via the keyboard are hidden.

Beware that different keyboard layouts have it in different spots (e.g. I always forget where the "¦" is hidden on an Belgian AZERTY keyboard (- I hate those, and always fight to get a US QWERTY only - ), but well younger people never learned that computers should have different keyboards from localized typewriters.

For any baffled users of OSX Tiger the location is:

System Preferences > International > Input Menu

...

Thanks for that correction Samizdata!
Don't know where International fell through but glad you found it ;)