This is a handy plugin even though a lot of themes are responsive (and, actually, even themes that are billed as responsive may not be flawlessly so depending on the platform).
I have this plugin installed on many of my sites -- even those with responsive themes! That's because there can be situations when even a responsive theme's small-screen layout doesn't look as good as a simple non-responsive one's..."WP Themes by Screensize" is one of the most useful free plugins I know of and it definitely works with 3.5.1 despite the lack of continued development. But hey, it's simple and lightweight so it should withstand many more updates to come!!
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