Sparrow, the current email client darling for OS X can be used in several different interface configurations depending on your preference, task and screen real estate.

The message list is the central window and messages can be opened in individual floating windows or as an panel attached to the message list. It can also be collapsed entirely to stay on top of emails in a minimal UI.

While there is a keyboard shortcut to toggle the message panel Sparrow also does something clever if you’re using a mouse — it lets you open and close the panel using the standard forward and back buttons on any mouse that includes them.