I went to add a mail filter rule to Apple Mail in Tiger, and the new rule came up with a default rule for filtering messages where "Any Recipient" matched the recipient of the current message I was looking at. Beautiful! It gets better -- I went to add another condition, which I changed to a "From" condition, and it automatically changed the initial value of the condition to match the sender of the current message. I never had to type in a single value, switch window panes, copy or paste.
This is something library and language designers could learn from HCI--making the common cases easy. (The Perl-mongers know it.) It's up there with progressive disclosure.