The Google Reader Minimalistic Greasemonkey script strips the header and makes the sidebar collapsible in GReader to maximize reading space (much like this GCal script). The developer says:
The end result is a screen that actually fits on my laptop a lot better, especially when you collapse the left hand navigation bar (with the arrow).
Knowing the right keyboard shortcuts to get around GReader is essential once you have this style installed, since you lose much of the visual navigation. UPDATE: As many of you have pointed out, this feature was added to Reader in the rollout that brought us the long-awaited search box. Sorry for the duplication of effort.