Where an organization wants to manually create tasks that have standard subjects, the “Subject Combo (New Window)” button to the right of the subject field in the new task UI is the obvious mechanism to use. The first time I searched for how to customize that list I failed to find the solution, but looking again today I did find it.
A clue is in the help:
Combobox Field Type
A combobox is a picklist that also allows users to type a value that is not already specified in the list.
The task subject field is modeled as a picklist field but presented as an editable text field and the picklist values are presented in a pop-up window that sets the selected value back in the task subject field when an entry is clicked. So to customize the task subjects you just need to customize the picklist values via Setup -> Customize -> Activities -> Task Fields.
Here is an example of the result:
While this Combobox Field Type isn’t conveniently available in Visualforce, it does look like the pattern could be copied by using apex:inputText rather than apex:inputField and with the values for the combobox window obtained in a custom controller from the DescribeFieldResult.getPicklistValues method.