Drop script backgrounder is a freeware app that turns an applescript compiled as a stay open application into a background only app. It does this by editing the "plst" resource. It can also edit this resource in other carbon apps if it exists, but the results of doing that may be unexpected. OSX is less strict about background apps opening windows, they will not crash the machine if they do so like an OS9 app will, but you will not be able to access any menus even if a window is open.
MacOS X 10.0 or later