If you take a computer along and you plan to connect to the Internet, you might need a Modular Telephone Adaptor for Greece since many older Greek telephone plugs are almost certainly different than the ones you use in your country. These older outlets are a rearity today (2010), but you should double-check, especially if you are staying in an older hotel/room.
The new standard telephone jacks (RJ11) are installed for all new telephone connections, but many older homes and hotels still utilize the old jacks. You can buy an inexpensive adapter to connect your RJ11 telephone cable to the wall outlet. If your hotel has the newer outlets and offers the ability to dial out directly from your room, all you need to do is to unplug the telephone from the wall and to plug in your computer.
SinceI originally wrote this page Greek hotels have adapted to the demand for wifi and I have found that most of the hotels (even obscure ones) have wifi available for guests. Some are free, some would charge, but they all vary in their strength of signal. In addition, you can find plenty of cafeterias with Internet connection in almost every city in Greece, so you can always check your e-mail, or send messages to your friends for a small fee.
If you take your computer to Greece, make sure that your computer can handle 220V of electricity before you plug it in a Greek outlet, or you might do a great deal of damage to your system. Check with your computer manual to see if you need to manually switch to 220V.
Also read about how connect to the internet in Greece