The Scythe Kozuti SCKZT-1000 is a really compact low profile heatsink that conforms to the 1.5U standard and measures just 40mm from tip to toe! That's not a lot of space in which to cram a heatsink capable of handling 150W heat loads... but Scythe manage it with average thermal results - impressive for such a low profile heatsink really.
Low profile heatsinks are classed in one of three different sizes; 3U which is 110mm or less in height, 2U which is 75mm or less in height and 1.5U which is less than 45mm tall. Heatsinks that conform to 1.5U are suitable for compact HTPC's, miniITX chassis and so called "book-PC" cases.
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Original URL, circa 2011: http://www.frostytech.com/articleview.cfm?articleid=2614