Marking all read in Mail.app
I've always been annoyed that the only way to mark all as read for a mailbox was with a contextual menu in the sidebar - lets change that.
I use Butler as my quicklaunch keyboard macro tool - but you could use your favorite one to run the following script while reading mail.
tell application "Mail"
get message viewer 1's selected mailboxes
repeat with thisBox in result
set read status of (messages of thisBox whose read status is false) to true
I find this almost critical for getting through mailing list traffic.