What is your aim?
If it is to design a garage, unless you already know a particular design program, the easiest way is paper, pencil and thinking.
This is a very valid point.
I have worked a bunch w/ AutoCAD in my career and very strong at plan reading so I tend to sketch things out on paper and clean them up in CAD. Of course I'm very much a visual guy, so having a marker board and the ability to draw something on the fly is priceless for me.
That being said, I have tried several of the suggestions previously listed and will share my thoughts:
1. Pencil & paper - Truly the basic where it all starts. Limitations can be having to re-draw and/or problems scaling objects. Some of this can be eliminated by using graphing paper.
2. Excel - More or less electronic graph paper and good for basic stuff. Revisions can be easy or hard, depending on what you are doing. The problem of getting items positioned in your garage to scale is still a problem.
3. Grizzly Site - Pretty cool actually with a decent library of some commonly used shop/garage items. Easier to scale and make changes. For me, the biggest downfall is it's an online program and doesn't allow me features that make the process go faster.
4. Sketchup - Lots of guys have used this with good success. I admit the 3D aspect is very cool. Myself I find it tedious to be precise, and for most projects, doing something in 3D isn't really needed. You can spend a lot of time designing and then finding the model objects and getting them to the proper scale/dimension. But for the price of free, it does a great job.
5. DraftSight - As someone mentioned, it rivals AutoCAD LT and is a very good basic CAD program. It offers tons of flexibility and customization, and things can easily be scaled as needed. Also, I like drawing in layers, so I can quickly toggle on/off a line to easily show/highlight electrical, plumbing, etc. If working with outside engineers/designers, they will normally give you a DWG or DXF file that can be opened in this program. The downside would be that it has a steep learning curve upfront if you've never operated a CAD program.
6. Solidworks, etc. - True 3D modeling software that is excellent in its own right. Complete overkill for the common DIY guy that doesn't already work with this software. Plus it's expensive and has a steep learning curve as well. Arguably one of the best programs out there in this category.