Phlegm, GM Level Editor

I suppose that would work. Does this support adding your own code?

Phlegm only saves backgrounds and objects in a file and loads them.
You have to write your engine yourself.

If your engine supports that, no problem. Phlegm just saves the ID of the object. The code of the object remains in the object. If your object's code includes creating more objects, it will also do that when loaded from Phlegm... If your object opens the CD drive, it will still do that when loaded from Phlegm. Phlegm only loads object IDs, nothing else.

Phlegm does not support creation code for objects, though, in case you wanted to know that. Instead you can store a number of integer arguments and so far one string argument for each object.
The reason for the creation code abscense is both security and filesize.