I have been using Google Hangouts for IM until now, but recently became interested in an app that would ensure privacy of my conversations.
I have looked at several solutions, however, there doesn't seem to be a clear leader amongst them. There has been talk about Telegram's crypto protocol being broken, while TextSecure seems to only be available for Android. Desktop client is a must for me.
Now, take this all with a grain of salt since this is all just HN celebrity-worship / appeal to authority, but I don't really know what else to go on, not being a security researcher myself...
That said, as far as I know TextSecure is Android only, as you say, but I believe I read that both a desktop and iOS version are under active development and nearing release.
Wasn't it recently partnered with WhatsApp? Does that mean if you use WhatsApp between two recent android clients it's encrypted? If so, WhatsApp might be an option for you.
But it doesn't seem like that's the case, though, since I've used TextSecure and there's some (necessary) complexity in the interface to generate and share a key, as well as notifications about whether your messages are loaded in memory unencrypted. I haven't noticed any of this with WhatsApp.