Power for my US laptop in Europe
You could purchase the "Apple World Travel Adapter Kit" from an Apple Store or from Apple.com before you leave. I do not know if this kit is available overseas (not the US). $39

EDIT #1:

You could also purchase this adapter for only $9. It's much cheaper, and appears to work in Greece. $9

EDIT #2:

If you want just the piece that goes into the existing charger, check out this US to Europe Converter. $5