Skype – make free calls, IM & video chat online
Skype is a multiplatform software that enables users worldwide to communicate online via voice, video & IM. It's currently the biggest and, in my opinion, the most user-fiendly software of it's kind "responsible for 8% of global international calling minutes, and with its users making 3.1 billion minutes of calls to landlines and mobiles in the third quarter of 2009".