Allow the user to set windows (e.g. notecards, IMs, inventory, and code) to "floating", making them a separate window apart from the SL interface, so users can minimize them to the Windows task bar, or drag them beyond the bounds of the SL window - especially useful for those with multiple monitors, to be able to put code/notecards/etc on another screen and have one dedicated to SL.
This is an excellent idea, which gets my full support. I run SL at 1600x480 so that I can have chat, IM etc at the sides not obscuring my view of the world. This means that I waste a lot of GPU time rendering the edges of the screen, especially since SL insists on unreadable transparent windows with no option to disable this.
This proposal is basically the same feature as "Desktop Windows" in mIRC.
An alternative method would be an option to have the 3D view in a window rather than fullscreen, then it could be put in the middle with windows arranged to one side.
Also related, it would be nice if an option that mimics the behaviour of the XMouse PowerToy for windows (and also the default behaviour on unix systems) whereby the window that the mouse is pointing at automatically becomes the active window. I am used to working this way and constantly end up filling my chat window with eeeeeeeeeeeeeeeeeeeeeee because I forgot to click before trying to move.
Lastly, it would be nice if there was a text input box on the chat history window, like there is on the IM window.