Plugin fixes positioning of the selected widgets, when the page is scrolled down. You can "fix" any active widget with two clicks. No problem to "fix" more than one widget, but remeber about browser window height!
License: GPLv2
Plugin page: http://wordpress.org/extend/plugins/q2w3-fixed-widget/
Screenshots: http://wordpress.org/extend/plugins/q2w3-fixed-widget/screenshots/