Whenever I start working on a new project, I generally default to Github and start researching which libraries or frameworks can help me quickly complete a demo or a prototype. When I can't find something on Github, here's where I go next.

Open Source

• Alcatraz - xCode package repository
• Ruby Gems - Ruby package repository
• npm - NodeJS package repository
• pip - Python package repository

Not Open Source

• Binpress - Binpress is the marketplace for commercial open-source projects.
• Codecanyon - Code for sale.
• Themeforest - This is really great for wordpress themes.
• Creative Market - Ready-to-use design assets.