- Autocomplete: suggestion lists and type-ahead functionality for user interactions involving text-entry
- Calendar: graphical date selection
- Charts: beautiful charts (line, bar, pie, etc.)
- ImageCropper: crop images!
- Rich Text Editor: highly configurable text processor
- Okay, really, the feature list goes on forever
I think open source projects with large innovative companies behind them are essential to progress in web development standards and more powerful programming tools. YUI is an excellent example of such a project that should be used by all web designers. Of course, there’s always a learning curve, but the long-term benefits far outweigh the costs.