I'm looking for a way to create a floating window in ArcMap. To give an example just look at the window of the Identify tool.
Floating means that it stays in front of the map document at all times and that the user can continue to work with ArcMap. I know that the interface IDockableWindowDef can be used to create dockable windows which can also float, but I don't want them to dock. To my knowledge it's not possible to prevent forms created by IDockableWindowManager from docking if e.g. the user pushes them to the border of the ArcMap window.
The solution was to search for keywords like child window and MDI. HTH
The problem's solution seems to be as easy as @llcf's answer:
MyForm form = new MyForm(); form.Show(NativeWindow.FromHandle(new IntPtr(m_application.hWnd)));