Here’s a very simple, very short, and yet (to me, anyway) very welcome hint about
OS X 10.5. In the new version of Mail, the sidebar folders—Mailboxes, On My Mac, Reminders, Smart Mailboxes, etc.—can now be rearranged. Just click-and-hold on any folder, then drag it to a new position. You don’t even have to have Mailboxes at the top of the list; it can be moved elsewhere, too. I have a lot of IMAP accounts, and each clutters the sidebar with a needless folder. While I can’t get rid of the folders, I can at least move On My Mac up above them, making it easier to file emails via drag-and-drop.
As a bonus, here’s a related hint on the RSS folder in the sidebar. If you’re not using Mail’s RSS reader, you might think you’re still stuck with that RSS folder in your sidebar. You’re not. To make it vanish, open up the RSS folder, hold down Shift, and click on each listed feed. Then Control-click on the selection and choose Delete Feeds from the pop-up menu. Confirm when asked, and when all of the feeds are deleted, the RSS folder in the sidebar icon will also vanish.