I am looking for documentation or sample code on how to build a standalone
GUI designer.

The basic functionality is

-- A tool box
-- A canvas that controls can be dropped onto and be (re)positioned and
viewed at design time.
-- Property Explorer to set the various properties of a control

As this will eventually be used in a commercial product, please limit
responses to tools/libraries/SDKs that are freely distributed.

Any help is appreciated.

Dave