The most important part of design is sitting back and thinking about the impact your choices will make. What will a given element say about your product? How will it affect functionality? If you can ask these questions, you can achieve great design. It easy to be distracted by glossy aesthetics, but a simple, well-thought-out, polished, application is what your goal should really be.
So pay attention to every detail.
No pixel should go unexamined. It sounds hard, and at first it's tedious, but it quickly becomes habitual. You start to look at the corners of boxes, or unfocus your eyes after you read a logo.
If you're going to take this one step further, learn about typography and negative space. Specifically vertical rhythm and the use of 6.
Just like you can't "hack" a well-designed API, you can't "hack" good design. It takes effort, attentiveness, and the willingness to try ten variations before you choose an option.
It includes its own great list of other resources, including A List Apart and Thinking with Type.
I think a vast majority of websites ignore even basic principles like spacing, negative space, readability, and rudimentary concepts about typeface pairing.
I would add these to the list, which have been essential to my learning iOS design: http://pttrns.com http://www.teehanlax.com/blog/iphone-4-gui-psd-retina-displa...
With all that, if you don't like the list having things I haven't used, why recommend that I add other sites I haven't used? Not being an ass...just curious
Also, I'm very much interested in design. And I by no means think it's a joke. Notice at the end I say this is no substitute for professional design work...it's simply a medium.
Here's a few more that I've found useful:
http://iconfinder.com (or the similar http://findicons.com)
http://365psd.com (free photoshop file everyday)
http://www.colourlovers.com/ (great for picking a color scheme)
http://graphicriver.net (really cheap graphics)
http://themeforest.net (themes)
Without you having to hone your design skill if you are not interested in it.
It was #1 on HN for a little while a year ago, and I still get positive comments on it. Hopefully it will be useful.
Are there any recommended books for backend developers / hackers / programmers, about introduction to design?