IMSI/Design's DoubleCAD XT is a compact CAD utility for Google SketchUp. Based on the developer's professional CAD product, DoubleCAD XT is similar to other slimmed-down CAD packages, but it offers more than most of them, for less: it's free.
DoubleCAD XT's interface will be familiar to many, even those with no CAD experience, since CAD tools generally look and function much like photo editors and other typical graphics utilities, and DoubleCAD XT is based on pro software. It's well designed, as you'd expect from design software. It's easy to use if you have CAD experience but easy to figure out if you don't. For a free tool, it offers basics and extras too numerous to list entirely, but a few are: Enhanced .SPK compatibility with SketchUp, layers, a 3D-to-2D flattener, a brush editor, and automatic scaling. DoubleCAD XT is a great choice as a .DWG viewer for those with older AutoCAD or AutoCAD LT versions that can't read the latest formats. It's also handy for those who need to import or otherwise view and utilize CAD images for other purposes, such as presentations or digital publishing.
DoubleCAD XT is certified for both Windows XP and Vista, but not yet in Windows 7. It downloads as a Zip file, but at nearly 58MB, that's not necessarily a bad thing. It also requires you to enter the provided serial number--not too many hoops to jump through for such fine free tools. Highly recommended.