Here is the small list of links that I always find useful to have in my bookmark. All those sites and articles are valuable addition and good source for Magento knowledge. Some of them are from official sites and some are from experienced Magento developers that love to shear knowledge.
Magento ofical knowledge base: http://www.magentocommerce.com/knowledge-base/
Extensionsmall Blog: http://www.extensionsmall.com/blog/