TurboCAD Learning Edition
(Windows)

As I use the full version of TurboCAD, this software should provide the greatest level of compatibility. It will only work in 2D (all these designs are in 2D only) and seems to provide extensive editing functions - it seems the 2D limitation is the only restriction.

This is not a TurboCAD page so perhaps it will not last long?

Autodesk DWG TrueView
(Windows)

This free Autodesk (tthe people that make AutoCad) viewer will open only .dwg files, but allows measurment and mark-up.

Autodesk also offer another viewer (Express Viewer) which will only open .dwf (drawing web format) - but almost no-one sems to use this format, so the viewer's not much use.

The Autodesk site is reorganised at intervales and these links may not take you directly to the relevant page. The pages can be found fairly easily, but please note that Autodesk steers you towards the less-than-useful .dwf viewer - to get the .dwg viewer, you need to look via their AutoCad pages.

eDrawing Viewer
(Windows)

This free viewer is provided by the company that sells Solid Works. It reads DXF and DWG as well as many other formats, including their own web format, 'eDrawings'. Works nicely - and has mouse wheel zoom control, which I now think is essential!

ProgeSoft DWG Viewer
(Windows)

This free viewer is provided by the company that sells IntelliCAD. Roger reports that it opens the .dwg files OK. It allows viewing, measuring and printing.

Bentley View
(Windows)

Paul from Oz reports that this free viewer opens all files, even when others don't. It handles .dgn (Microstation) files as well as .dwg files. The only problem is the 66Mb download size!

QCAD
(Linux/Unix/Mac/Windows)

Dennis Opfermann reckons this is a good basic CAD program - the free version only runs for 10 minutes at a time, but it's only $26 for the full version. If you are of the Linux or Unix faiths, this is as near as I've seen to free!

A9CAD
(Windows)

This is a freeware basic 2D CAD program (also available from Tucows). If anyone tries it, please let me know how you find it.

PowerCADD Viewer
(Mac)

Dave Nathanson says that the free viewer at this site will open the .dwg files if you are blessed with a Mac.

Cad Standard Lite
AlleyCAD v3.4
FreeCad
(all Windows)

Three free CAD programs suggested by Dee Bee in Pennsylvania.  FreeCad is 3D and has motion simulation capability.

Solid Edge Free 2D Drafting
(Windows)

A free 2D CAD program from one of the big 3D software houses - it is intended to allow the professional maintenance of old 2D drawings, so it is likely to be a gold-plated product - I haven't heard yet from anyone that's used it, but as a 250 Mb download, it sure isn't small!

JustCad
(Windows)

Another free CAD programs, this time suggested by Kerry.  From its description it looks to be very basic, but then for a CAD beginner that's a virtue!.

Google Sketchup
(Windows)

SketchUp is not really a CAD program - it's more a 3D modelling and visualisation program - but it is the easiest 3D software to learn to use by a big margin. The downside is that while it's very easy to get started, it can be a problem to get it to do exactly what you want.

The free version of SketchUp lets you import DXF files, but it won't let you export them - for that you need the (expensive) Pro version.