[latchkey]

You don't need this. Turn on the 'Labs' feature called 'Preview Pane'. I've been using it for a while now and after a bit of getting used to it, it is great.

[AdamGibbins]

I don't think this does what you think it does. Admittedly its not greatly named.

The feature in GMail allows you to have a pane with your message in, similar to the feature seen in most desktop clients.

This extension allows you to minimise the header in the gmail window. See the video on the link.

[justinschuh]

I was going to point this out as well. Between the "Preview Pane" and "Nested Labels" labs I'm very happy with Gmail.