On the heatsink test bench today is Xigmatek's Prime SD1484 heatsink which features a few notable changes to the standard, stock tower heatsink Xigmatek offers. For starters, the edges of the aluminum fins are squished slightly to form a blunt knife edge to reduce air flow resistance - Gigabyte tried this in 2006 with good results - Xigmatek have applied a 4mm chamfer to both edges of the fins. I'm not sure how effective this will be in real world terms, the thickness of the aluminum fins is 0.5mm to begin with while this added process only reduces that to ~0.3mm.
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Original URL, circa 2011: http://www.frostytech.com/articleview.cfm?articleID=2613