The Yahoo Design Patterns Library is a must-see if you are into web design and web development. There you can find answers to common design problems, such as how to correctly implement tabbed navigation, date-pickers, sign-in wizards, etc.
Each section details the problem faced, when to use a certain UI solution, and the proper ways to implement it, considering both usability and accessibility issues. You can also join the forums to further discuss each pattern and suggest ways to improve it, or even suggest new patterns, if you think something is missing. Either way, this is a great resource that you should probably use extensively when working on web projects. Keep it at hand at all times!