First, I'm not interested in doing this professionally. I am a web developer, a coworker of mine recently left for Spotify and said he will be working mostly in JavaScript for the Spotify Desktop app. He said it uses "Chrome frame" and everything inside is done like a web app (HTML/JS/CSS).
As a web developer who never built anything for Desktop, this is great news. If I can use the technologies I already know and implement them inside some sort of a "frame" and still be able to build a windows or better yet cross platform app.
I know I didn't mention anything about the database, but even a simple hello world desktop app with web technologies would be great to get going.
So how does one go about this? Exactly what do I need/need to know?
Update
My previous answer is now outdated. These days you would be crazy not to look into using Electron for this. Many popular desktop apps have been developed on top of it.
You may start with Titanium for desktop dev. Also you may have a look at Chromium Embedded Framework. It's basically a web browser control based on chromium.
It's written in C++ so you can do all the low level OS stuff you want(Growl, tray icons, local file access, com ports, etc) in your container app, and then all the application logic and gui in html/javascript. It allows you to intercept any http request to either serve local resources or perform some custom action. For example, a request to http://localapp.com/SetTrayIconState?state=active could be intercepted by the container and then call the C++ function to update the tray icon.
It also allows you to create functions that can be called directly from JavaScript.
It's very difficult to debug JavaScript directly in CEF. There's no support for anything like Firebug.
You may also try AppJS.com (Helps to build Desktop Applications. for Linux, Windows and Mac using HTML, CSS and JavaScript)
Also, as pointed out by @Clint, the team at brackets.io (Adobe) created an awesome shell using Chromium Embedded Framework that makes it much easier to get started. It is called the brackets shell: github.com/adobe/brackets-shell Find out more about it here: clintberry.com/2013/html5-desktop-apps-with-brackets-shell
CEF offers lot of flexibility and options for customisation. But if the intent is to develop quickly node-webkit is also a good option. Node-web kit also offers ability to call node modules directly from DOM.
If there aren't any native modules to integrate Node-Webkit can offer better mileage. With native modules C/C++ or even C# it is better with CEF.
You can build Javascript apps with Adobe AIR… http://www.adobe.com/products/air.html
NW.js
(Previously known as node-webkit)
I would suggest NW.js if you are familiar with Node or experienced with JavaScript.
You can find the NW.js repo here, and a good introduction to NW.js here. If you fancy learning Node.js I would recommend this SO post with a lot of good links.
It seems the solutions for HTML/JS/CSS desktop apps are in no short supply.
One solution I have just come across is TideSDK: http://www.tidesdk.org/, which seems very promising, looking at the documentation.
You can develop with Python, PHP or Ruby, and package it for Mac, Windows or Linux.