The easiest is this: dial91.com. I have been using this service to call my buddies in India for a few years now. The quality is amazing! It costs me about a cent a minute. The calls to UK are 2 cents a minute from the US. I find this service the easiest to use because, they offer me local US numbers that I can dial into, then speed dial from there (they also have a smart phone app). The speed dial numbers can be setup on their website under your account (and your phone number can be registered as well - all this is a one time effort of a few minutes). So if I have to call buddy X then I call one of dial91's US numbers saved on my phone and then dial the speed dial code for X. Thats all.
I personally hate to be in a contract, but, I have also heard good things about Vonage.
I usually like to have quality in my phone calls and am willing to pay for it. In my experience I have always found skype calls to cell phones lacking in quality (for video calls with even with I being on a 30 Meg line and the other party on a 10 Meg line ! and a phone d/l u/l speed of 4 and 1 Meg).