The most important thing about designing in open source is communication. Designers are not only designers who finish a concept and »hand it off« to developers who implement it – that won’t happen. The key is continuous dialog with the people who could implement it, small steps, incremental improvements and documenting your designs clearly so it’s understandable why the changes you propose are better than the current state.

• pen & paper for mockups. No hi-fi photoshop work needed – when we have non-interactive mockups for an interactive medium, pen & paper does the trick.
• design discussion on the issue tracker of the project. For example at GitHub – get an account, identify design issues, participate, and open new issues. Establish a »Design« tag on the tracker to group these issues, for example like in the ownCloud issues.
• if you want to dive into the code (awesome!) just check on the project page how to set up a development environment and get going.

• Creative Commons search
• Wikimedia
• Flickr
• Youtube
• SoundCloud
• Vimeo Music Store has CC and purchaseable icons

• The Noun Project CC and purchaseable icons
• IcoMoon CC icons and free to use icon-font creation software.

• The League of Movable Type
• Open Font Library
• Google Web Fonts

• Bootstrap Fully featured mature HTML, CSS (LESS) & Javascript
• Foundation Fully featured mature HTML, CSS (Sass) & Javascript
• Rebar Simple lightweight HTML, CSS (LESS)

• First Rule of Usability? Don’t Listen to Users by Jakob Nielsen (2001)
• 10 Heuristics for User Interface Design by Jakob Nielsen (1995)
• 19 Principles of User Interface Design by Joshua Porter
• ten principles for good design by Dieter Rams (1970s)
• Usability in Free Software
• A Beginner’s Guide to Finding User Needs
• Open Design Now book
• How to get designers (or anyone) to work on your open source project recent article on opensourcedesign.is
• Don't Make Me Think
• The Design of Everyday Things

(Focused on HTML, CSS, Javascript.) Ideally, to be able to implement your own suggestions. But also to better discuss with developers and be understand why and how certain things work and don’t work.

• WebPlatform web development resources
• Mozilla Developer Network for great web dev documentation
• Unheap a tidy repository of jQuery plugins
• Transitional Interfaces, Coded for good animations
• Codecademy with interactive tutorials
• Try Git also with interactive tutorials
• code.org
• Code School learn even more
• Egghead to learn the AngularJS framework

Learn about design concepts!

• Hack Design Free design lessons by inspirational designers
• Design Lab Design course with mentorshop and projcts
• Voice and Tone Learn about writing good copy

• Libre Graphics magazine print magazine
• Libre Projects collection of nice open source web apps
• Libre Graphics World
• Libre Graphics Meeting conference
• Transifex an open translation platform lots of open source projects use
• Indiegogo crowdfunding platform, also useful for funding open source projects
• Beautiful Open Beautiful sites for Open Source projects.
• Design Open Src A list of awesome designers willing to work on Open Src projects, created by @clintonhalpin
• Open Design and Hardware Group. OKFN open design and hardware group.