With Appcelerator Titanum, you can use HTML/CSS/Javascript/PHP/Python/Ruby to deliver native desktop and mobile applications.

Titanium has 2 SDKs:

• Desktop SDK:   which “packages up your application source code (HTML/CSS/JavaScript) with a heavily augmented build of the popular webkit open source web browser engine.”
• Mobile SDK: which does not use browsers. in fact titanium mobile applications are run against a standalone JavaScript engine which invokes native APIs.

You can build and run Android and BlackBerry applications for free, but for iOS, only developer account is free and you will need to pay a $99/year fee for the iOS developer program.

in addition to two SDKs it has a handy desktop application too:

• Titanium Developer is a desktop application you will install on your computer to allow you to create, run, and package Titanium Mobile or Desktop application projects.

Resources:

1. IBM DeveloperWorks has a simple hello world tutorial for building Mac OS X platform:  http://www.ibm.com/developerworks/library/os-titanium/index.html
2. Getting Started with Appcelerator:  http://developer.appcelerator.com/guides/en/getting_started.html
3. Application Demo:  Kitchen Sink.
4. Showcase gallery: http://www.appcelerator.com/showcase/