When you edit an item in the web interface, combo box type of fields now have an enhanced search box.
Example: in the Contacts web app, when you Edit a Contact, and select the Company field. You can search the company by entering a substring of the company name. The combo box will filter down to the list of companies containing the search string.
Or when editing the linked Contacts field of a data item (task, project, opportunity,..) you can search for the contact by first or last name.
This is especially useful when you have a large number of items in the combo box and don’t know exactly the name you are looking for.