Koh Chang has many internet cafes that offer internet access from 1-2B per minute via satellite or phone connections. Increasingly hotels, bars and restaurants are providing free wireless (WIFI) internet access for their customers. So, if you've got your laptop, Skype and a headset its, "money for nothing and your kicks for free...."
There is a brand new (September 2006) government post office 2km from White Sands Beach. Postage rates are reasonable. A small postcard to any country is 12B and large postcards are 15B. Air-mail letters start at 14 Baht and increase depending on destination and size. (The Guide Online Offices are now located in the Old Post Office building!)
Local and national phone calls can be made at several of the resorts and businesses on the island (10B/min local or 30B/min international are typical prices). There are international phone terminals on most of the main beaches. All the convenience stores sell phone cards which offer good rates to International destinations from both mobile and land line phones.
If you are travelling with your own mobile phone you will be either pleased or disappointed to learn that the island has almost complete signal coverage.