This is the same as the ProcessWire Text field (FieldtypeText), but enforces that values stored are unique, site-wide.
To install, place FieldtypeTextUnique.module in /site/modules/FieldtypeTextUnique/, and then install from the Modules admin screen.
To use: create a new field and choose "TextUnique" as the type.
To use on an existing field: edit the field (Setup > Fields > field) and then change the type to "TextUnique". Note that the values present in your existing text field must already be unique, or the change will fail.