Make a relationship based on the "other field" Then in the table you want your list to be from have a key and item you want. Then base the valueList on it with relationship
here is a quick example
What Jared says will help you out for a simple FileMaker solution.
If you ever want to get fancier say with drill down menu's, etc., there is a plugin from CNS called "CNS Menus" that is pretty nifty.
Jared's solution works well if you want values from another table in the dropdown list.
If you want other values, e.g. for actions like "delete", "edit", "go to previous record", etc,
you might consider a formula-field that calculates the desired values of the list
based on the values of these other fields.
You can then define the value-list based on a relationship to itself
over the record's id to that formula-field (just as Jared said),
to get the value-list to display the values defined by the formula.