Uncomplex has tweaked Mailplane, its WebKit wrapper for Gmail, with a handful of minor fixes and improvements. Picture optimization in outgoing messages can now be controlled via AppleScript, dragged messages now disappear as expected, dragging images onto Mailplane's icon now optimizes pictures reliably, and keyboard shortcut functionality broken by Gmail changes has been restored ($24.95 new, free update, 19.9 MB)
Compare More Easily in Apple Mail
In Apple Mail, if you need to work back and forth between two different views of Mail's mailbox contents, you can do so quite easily. For example, you might want to look at a mailbox holding all filtered-in sales orders from the past week while also looking at a smart mailbox showing unanswered customer questions.
To avoid constantly clicking between mailbox views and losing your context each time, choose File > New Viewer window to get a second window and then arrange each window as desired.