Unicode Greek Inputter
It's devised by James Naughton from Oxford. This is how he describes it:
The Unicode Greek Inputter is a small utility for producing polytonic Greek text with an ordinary English keyboard, using a betacode-like input method. Paste your resulting text into Unicode programs such as OpenOffice, Word 97 or 2000. Save the utility as an html file for use offline.
I've tried it out and it is an excellent resource. The hint at the end is very useful -- plan ahead now for when it vanishes from the web!