DoodleChimp is a browser-based webpage markup utility for drawing directly on a webpage in your browser and saving the mark-up to be able to send a link to someone else so they can see exactly what you drew without special plugins or applications installe
FASTTAC delivers rapid access to critical construction documentation in the field and in the office for general contractors, architects, engineering firms and owners/developers.
SimonSoft provides CAD Viewer with AutoCAD 2004/2005 support including Markup, Redline Tool. Components of ActiveX and DLL for plugging DWG capabilities into your applications.