In the mailboxes list on the left side of the Mail window, there's a REMINDERS section with a To Do list. As far as I can tell, there's no way to remove the REMINDERS section or the To Do list. I never use these. I don't need Mail to remind me of things; that's what I use iCal for. They've just talking up valuable space in my mailboxes list. I have a number of mailboxes, and they were previously all visible in the Mail sidebar in Tiger, but now in Leopard the mailboxes at the bottom have scrolled off the bottom of the list so that I can't see them anymore, and I'm forced to scroll the list.