Going through list of plugins not active on my blog, I saw the Q2W3 Fixed Widget.
I remember Befor that I wanted to make a particular sidebar widget fixed when the page is scrolled down. After few googling, I finally settled for by this plugin.
This plugin adds a Fixed widget checkbox at the bottom of every widget. If the checkbox is ticked, that widget(s) become fixed.I hope You will Enjoy It.
Mind you, the plugin has a configuration page. I assume you know CSS. Setting it up shouldn’t be a headache. You can get the setup info from the plugin Authors Website . If you Have any problem, let me know in comment.