I'm working with a .NET, SQL Server desktop client server app that allows:

        Custom Tables
        Custom Stored Procs & Views (These can have data modification capabilities as well)
        Creation of custom data sources in the application (Inside or outside the 'live' database)
        Creation of custom grid forms based on custom data source or combining of custom data sources
        Creation of custom reports based on custom data sources
        Creation of custom logic script on data entry fields 
        Customizable data import functionality
        Customizable workflow process data entry and notifications
        Customizable selection and placement of data entry fields/controls on standard forms
        Detailed user security settings to all forms, data CRUD, app functionality and reports

Seriously, this company created a customizable beast. It's designed for a vertical market, but the other implementations could be limitless.