Better folders, which support drag & drop: is that even possible? I've tried three different folder widgets for Android 2.3, but I noticed none of them supports adding applications via drag &amp; drop, like the native folders do.

Is that even possible to do?

I guess it isn't, or those application would be pretty stupid not to support it, but you never know.
The only interactions presently supported by app widgets on the home screen are simple click events.  So the answer to your question is no, drag &amp; drop isn't available with widgets.  The reason the native folders can do this is because they are just part of the launcher activity so can be programmed to do more complex things than the widgets can