Hm, I am fininding myself wishing that we had communitywiki
After seeing this question: What is the most common user interface in the world? I thought it would be educational to not limit the discussion by trying to single out the most common, but instead discuss the many common interfaces that we (read: mankind) interact with daily, and how they can be improved.
So, what are the most common UIs/UI components? What are their strengths and weaknesses? How can they be improved?
Combo boxes are abundant through out the web and in desktop software.
Pros: Good for allowing a user to have a list of choices while requiring minimal space when not in use.
Problem: Becomes unwieldy and hard for a user if the list of choices is too long.
Potential Solution: Autocomplete Combo Boxes, using text matching on typed user input to narrow the list of possible choices and enable easier selection.
Obviously I didn't invent the Autocomplete combobox; note that answers don't have to be improvements you invented.