Skype is a great way to communicate.. I prefer it over other programs as it better suits a travelling person.
Open a Skype Account. Go to www.skype.com
(Computer->Computer) Search for your friends online. Add them by (Contacts / Add Contact). This is a free service.
(Computer->Phone). This is called SKYPE OUT. You can dial any fixed or mobile phone where around the world if you add credit in your SKYPEOUT account (go to www.skypeout.com). Paypal and Credit card are supported. You can either add 10 EURO or 25 EURO at one time. Calls are cheap (depending on country), and are relatively comparable to IP cards. Remember to dial the country prefix!
(Phone->Computer). This is called SKYPE IN. This is a VERY convenient service. SKYPE IN (www.skypein.com)allows you to reserve a phone number in a country of choice. The cost is 30 EURO a year, or 10 EURO for 3 months. This allows you to hand out a local number to friends / work collegues in a different country (so they are charged only a local call), and when they call you, your SKYPE on your computer will ring (no matter where you are in the world!). This is handy if you want to have a number on your business card for a country that you do not reside. The only cost to you is the yearly rental of the number, and the caller is charged a normal phone call charge.