I’m liking the FancyBox implementation.

Designed for Google Reader, Simplify GR integrates tools such as Helvetireader and FancyBox to provide a clean, minimalistic reading experience.

The Simplify GR extension utilizes Helvetireader to reduce the clutter of the Google Reader interface by limiting the number of colours, buttons and social features normally present. When reading articles in Google Reader some entries contain the entire article, while others do not, requiring you to visit the originating website. Simplify GR makes this process more seamless by opening the originating web site in an clean panel that floats overtop of the Google Reader page. This panel allows you to read an article and return to Google Reader in an incredibly seamless fashion. Simplify GR takes its seamlessness even further by opening full articles loaded within this floating panel through Instapaper‘s text view–providing a distraction-free reading experience similar to Safari 5’s Reader mode.

All of these options are configurable within Safari 5’s Preference Extensions pane. You can enable/disable Helvetireader as well as enable/disable FancyBox or adjust how it works.