If you are talking about something that is completely freeform then it's another story. I personally use Illustrator for the few times that I do it but that's a bit expensive for using that feature alone.

For scaling it helps to put a tape measure along side the item before photographing. Then just expand the picture in inventor to equal the length on the tape measure.

I'm assuming you can do similar in other CAD programs.

Import raster image to Inkscape.
Use Inkscapes tracing tool and set fill to none and outline to black for example.
Save as svg.
Import svg to Blender. It should show as a path/curve.
In the path tools tab on the right set to 2d path and use extrude setting to pull the path to 3d. (Can't remember details as I'm not on my computer now )
Tweak your model and export directly to stl.

The gray values end up mapping to a length along an axis.
Therefore if you created a pure black&white image you should be good to go.

Of course, it depends on your 3d software having this feature.

For hand tracing, I imported jpg into Alibre.  I never quite figured out how to get the jpg  to a particular scale, so I always had to resize after I traced which was a bit of a pain.  I would think that most CAD programs would at least let you load a bitmap into the background or onto a plane to trace over.  It' s a pretty basic feature.  Mesh modelers should be able to do this as well, and export a dxf or something.

See the attached images, took a picture from a magazine - brought it into autocad for the trace - imported to solidworks and modeled the shape and variable radius fillet and output to stl for the print.  It's easy if you have the tools but I'm sure the workaround is going to be harder using other software.

Conveting raster to CAD can be tricky, because sometimes you get a lot of extra points that make the shape almost impossible to edit.  It also tends to create a lot of unnecessary polygons when you convert to STL.

It's a little more work up front than conversion, but I've found that loading the bitmap as reference and then tracing over it by hand with splines and such gives the most usable result.

I understand that for sure.  I interned with a company that did vinyl stickers about 18 years ago.  We would scan in art work and have to fix up all the extra points, since the vinyl cutters need nice lines and nice curves in order to cut.  What software would you use to do the tracing like this?

It's a little more work up front than conversion, but I've found that loading the bitmap as reference and then tracing over it by hand with splines and such gives the most usable result.

your better off learning basic 3d modelling.
thats essentially what your asking for.

I can do some 3d modeling, but some shapes I'm just not going to spend the time trying to get them right, when what I need is an outline of an existing part I could just trace onto paper.

This is not what I'm building, but imagine you have something with a unique shape, like the side view of a set of measuring calipers.  You would like to create a case for these where you have a solid and then a cutout that perfectly fits around the object.

You set the calipers on a block of foam 1/2" thick, and cut it out with an exacto knife.  Done.  That was easy.

What I'm proposing is to trace that unique shape on to a piece of paper with a pencil, and scan that in to get a base shape.  Then extrude that shape 1/2" cut into a solid in the model and get my outline.  Sure I could do a bunch of measurements and try to get all of the curves and everything proper, but I'm not going for 100% accuracy or full 3d replication, I just need the outline so it rests nicely in around an existing object.

I'm more interested in getting this process down because I see myself using it a lot for creating custom trays and holders for interesting shaped parts or household items.  When things are round or square it's easy to take a couple measurements, but then they contain a lot of curves , or curves that change it becomes much harder.  The effect I'm going for is from real world part to printing a mating part with very little time spend modeling.

I don't have solidworks or autocad.  I'm still in the process of deciding what software to use.  I'm thinking alibre is out of the question because the sales person rubbed me the wrong way.  I hate not being able to speak and getting interrupted when trying to explain my needs.

I'll play around with img2cad and see if it make sense for me.  It looks promising to get a scan to a 2d shape.  From there I'll just need a workflow to import that and extrude, I imagine many packages can do that.

If you're going to use autocad I can post a little lisp routine that sizes the raster image so you can trace it more accurately.

you draw a 2d shape in cad then "extrude" to create a 3d model.

Im currently making a acrylic table top for a wine barrel that has my football teams logo machined into it.
My teams logo is a hawk, as you can imagine drawing a hawk in cad was tricky so i searched for a jpeg to dxf converter.
using this i was able to create a 2d dxf from a image file which in turn i could use to create a 3d model of.

program is called img2cad its free
i understand this is not what your asking for but its close, you can use the 2d dxf to create your 3d model.
www.img2cad.com

attached image is the results i achieved using this.

I'd like to import that 2d drawing (scanned as a PDF or jpg or other raster graphic) into a 3d cad program to be extruded into a 3d part. 

Does anyone know of an application that makes this easy?

I was sort of able to do this with alibre, but it was about a 32.5 step process that involved a few other applications and truthfully I'm not sure I could repeat the process.

I'd like to be able to trace the outline of an object and create something that shape and size without a lot of measuring..  which becomes hard for none uniform parts...

Does anyone know of a workflow that makes this easier than the 32.5 step process that I stumbled on earlier?