MS Visual Studio has a great feature: it automatically suggests the units to add in using clause when you typing the code with refrences to absent standard classes.
using
Or maybe you can try this: http://www.epocalipse.com/products.htm
Unit Expert
"A freeware Delphi add-in that allows you to quickly open units and also add them to the uses clause."
I never tried this expert but it semms promising.