Yahoo! Design Pattern Library is great resource of best practice best practice patterns. I discovered it just recently and found it very interesting and resourceful.
You can find patterns related to transitions, carousels, pagination, tabs, grids, tool tips and many more. There is currently 50 patterns in the library to learn about.
As an addition, you cand download all patterns as Design Stencil Kit for Omnigraffle or Visio or as PDF, PNG, SVG or for Axure and InDesign.
UPDATE:
Just discovered UI Patterns library – looks, great!
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