The funds have to be distributed and overseen by the Greek government, and we all know how effective governments can be in using relief money and supplies in a timely and effective fashion.

However, for what it's worth, several European organizations have banded together to create an online relief fund which looks to me to be legitimate (always a worry at such times). They've set it up so that donors can pay by credit card. Quoth the website:
The money you donate via the website is directly deposited into the respective account of the Fund in Aid of the fire-stricken areas set up by the Ministry of Economy & Finance with a view to assisting the fire victims and restoring materials damaged.

I received an email receipt from them for my (modest) donation. So if anyone's interested, here's the website:

I Help Greece.com