Skype is one of the best apps when it comes to interacting one-on-one with friends. This software will let you look for anybody in the world if you know the skype ID.
Once you have connected with that person you can chat via text, or you can even make an call so that you can talk in real-time with that person, most time with high-quality sound. If your internet connection allows, you could even make a video call so that you can see that person while you are talking to him/her.
If you contact anybody that uses the apps, any call or text sent will cost nothing!
On the other hand, you can also make real phone calls to landlines or cell-phones, or even send SMS if you want to (costs vary depending on country of residence.) For that, you can either purchase a specific plan for unlimited calls in a certain country/region or you can buy credits that are deducted by call.
The possibility to have all this not only on your computer, but also on your cell-phone allows you to have this awesome experience anywhere you go.