I currently use Telegram (because it just works very nicely) and WhatsApp (because stubborn people insist on using it). Both can be used on the desktop via http://web.whatsapp.com and http://web.telegram.org. Telegram also has a native desktop client for Linux.
From the security standpoint, both seem inferior to Signal (formerly TextSecure). Is there any way to use Signal on the desktop as well? I guess that would break the end-to-end encryption but I would be willing to share the key between desktop and mobile.