Telegram Desktop is an easy-to-use, secure messaging and VoIP service. The program can be installed on multiple locations and devices, and will synchronize with all connected tools. It supports broadcast messages, huge chat rooms and can transfer any file type, up to 1.5 gigs each.
Note that the program requires to a working phone number.
Cross-platform with clients available for Mac, Linux, iOS, Android and as a web app. A PortableApps version is also available.