After my last couple of trips, I became a huge fan of ID Phone Cards
. You can go online to buy a pin and credits and they have dial-in numbers from just about anywhere... super-cheap, too!
I used that in conjunction with my cell phone, which with GSM, I have international capability, though in Australia I just bought a local SIM card. Still, calling card calls are way cheaper than making international calls on the mobile.
Also, look into perks at various hostels. Some places (like the Costa Rican place Joker and I have stayed at) offer free calls home every day. Some internet cafes have free international calls via VoIP. When I bought my YHA/HI card, I also got a calling card with $5 worth of international calling... There are deals around every corner!