Shhh History Blocker for Chrome
More About Shhh History Blocker
Shhh - history blocker allows you to temporarily disable storing visited pages in chrome history. Now you can do your "homework" and "other stuff" without fear.
Usage: click extension icon to toggle
UPDATE 0.3: added right click menu button that opens selected content(selection, image or url) in new tab while enabling shhh.
selection based context action performs google search