To just view DXF files, Autodesk (the AutoCAD guys) has a free file viewer at Autodesk - Autodesk DWG - DWG TrueView Download. If you want to modify a DXF file, then, like sqatch said, you need a CAD program or a vector-based graphics program. Inkscape is an open source (i.e., free) graphics program that will work with DXF files. It also has the ability to trace "flat" graphics files like .jpg so you can create your own DXF files.