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Electron, specifically...

Interesting. I haven't used it because they don't support Linux yet. (EDIT: I mean Slack, not electron)

You can build a wrapper (using a webview) of slack web with nw.js or electron... You'll get pretty much the same functionalities, including desktop notifications for the chats.

Spotify and Skype come to mind as well.

Is Skype considered start up?

Spotify uses Chromium Embedded Framework to serve a lot of UI elements via HTML/CSS/JavaScript[0]. Anecdotally, there seem to be a number of desktop apps moving in this direction.

[0] http://www.quora.com/What-is-the-technology-behind-the-Spoti...

It's still a desktop application.

Just built with web technologies vs natively compiled binaries, which is an implementation detail that shouldn't matter to the end user, in my opinion.