Programmers usually don't pay much attention to user interfaces and focus more on functionalities but users first judge the product by its appearance so it is necessary to make nice user interface for web, desktop and mobile apps.
Personally I don't like to spend much time for user interface design and sometimes try to look for some examples and then modify the existing design.
Does anyone know any good web resource with collection of user interfaces for web, desktop and mobile devices?
You can also have a look at the Yahoo design pattern and particularly the wireframe stencil kit that you can use to design your user interface: http://developer.yahoo.com/ypatterns/wireframes/
I use Balsamiq Mockups and their site has a good collection of mockups you can look at. Also UI-patterns and inspireUx are both really good resources.
This Design Patterns set by Chris Messina on Flickr.com seems to be good.
Design pattern libraries are very useful, but it can be a bit dangerous to look at them and "pick your favourites". This is not how good UI design happens. Different design patterns are useful for different contexts.
You may be better off by starting with a review of competitor products, and building up your own understanding of what works and what doesn't within your applications specific context.
For the web I recommend you to take a look at those links:
http://layouts.ironmyers.com/
http://www.webreference.com/authoring/style/sheets/layout/advanced/
http://www.code-sucks.com/css%20layouts/
http://developer.yahoo.com/yui/grids/
Not a web resource, but the user interface for BBEdit from Bare Bones Software (text editor, Mac OS) is the best I have ever encountered and is worth studying and emitating. What is learned from such a study can be applied on any platform, not just Mac OS.
"Best" = simple to use for beginners, intuitive gradual discovery of more advanced features.