Pixelmator - The eyedropper tool shows you what color you picked inside the pipette.
Chrome - Accent colour on most visited thumbnails is pulled from the site’s favicon.
/via Andrew Procter
pinboard.in - When changing your password, the new password text box uses a low contrast font and forgoes the typical type-your-password-again pattern.
/via Lenny Sirivong
Lion Safari - The scrollbar changes the color depending of the background of the current website. (dark or light)
Apple - Shows black and white buttons appropriately to help you select the color for your iPad 2.
/via Daniel Burka
Basecamp - Speech bubble is green if the item contains unread comments.
/vis Shaun C.
Google Mobile Weather - Background color changes according to the time of searching.