Pojo - When uploading an image, the picture is shown immediately with an upload progress overlay.
Google Maps - When zooming in to the maximum, the zoom in button merges with the street view button indicating that the next zoom level is with street view.
/via Toby Adams
Flickr — When uploading files, it shows percentage in the tab, so you can always see the load progress while surfing other sites.
Lion Safari - The scrollbar changes the color depending of the background of the current website. (dark or light)
Chrome - Notification bar overlaps with the browser UI, that way it can be distinguished from fake bars.
Mac App Store - The progress bar of installing an app from the Mac App Store changes its dimension separate from the icon, when magnified in the dock.
Chrome - Status bar shift down with the cursor when it is hovering a link near the bottom of the page. After a few seconds its hows the entire URL.
Windows 7 - If the task bar is pulled out to double height or more, Windows makes use of the extra space by adding the day and date to the time.
Apple - Menu drops down and bounces a little bit. Search bar expands intelligently once activated and shows different search results depending on what page you are on.
/Thanks to Renato for being the first to submit.