Cumulate Draw is probably more of a Visio replacement than a CAD replacement. It’s certainly not as powerful as, say, Autodesk Architectural Desktop. But if you don’t need dimensions accurate to an eighth of an inch and just want to sketch a quick floor plan, Cumulate Draw might suit your needs. It is free and available online. Your work can be exported to several different formats, including JPEG, PNG, and SVG.
Here is a quick office layout I created in Cumulate Draw. Click on the thumbnail to view the full-sized image.