I used www.floorplanner.com

It's free and allowed me to get a basic layout done quickly and easily.  Once you have your floor plan setup, then you can get into more detail with a 3d app.

This may be a bit too simple for what you are looking to accomplish but for folks that just need a simple tool, I find FP to be very useful.  When you place the little people (drop in tools) into your floor plan you can get a sense for the flow of the room and get an idea of what it will be like moving from tool to tool.

I personally think the best shop layout program is between your ears.

 i agree . there is no substitute for putting the tool in position and seeing will it fit etc. you can use all the software you want but its not fool proof. those software programs are great for large workshops with large machines spaced far appart. in the confines of a small cramped shop full of machines that are really close the program cant see every aspects of the machine and how it interacts with other machines

Guys, everyone needs an initial layout to work with. If nothing more one needs to know if the tools they get and the distances they need to work with will even fit in a shop of the size they have or are thinking of getting.

Of course one organizes the tools on how they work and of course it evolves, but I am the type that wants a plan and not just to throw a 100,000.00 or more of equipment in a pile and then try to make a project and move the tools around at that point. Plus, I have to have a starting point for my electrics, etc! I know where most tools where go and how my flow works, I am not a hobbyist that purchased my first tools. What I dont know is the exact layout of the air, water and electric in relation to those or exactly where to place the outlets to satisfy an inspector(I know every 4-6 feet etc at 18", every 72"- 48" off floor, etc, some in the floor), but I must have it on paper(in software). I can't just do that on the fly, the county wants plans and I want to play with it on paper(computer screens) and document my work flow in several configurations and be able to change them around easily, for that software is the way to go. Heck, I am not sure where to put the bathroom or what side the Cyclone add on goes or where I want my spray booth!

I do not  set up a shop in real life trying to move around 500lb plus tools on the fly and then try to get in the electrics lighting and everything else after the fact. Anyone starting a brand new shop should a starting plan(IMHO). I just wanted a suggestion on software to get my ideas down where I could see them.

So though I think many are trying to help,  some of the things being suggested are not realistic. I think trying a variety of configurations in the software, finally choosing the best and then after a month or two of actual work  tweaking it is the way to go. To just do everything with no plan is not for me at all.

I just decided to use Sketch Up and everything is in 3D. Way overkill, but it allows me to change stuff on the fly and is very accurate. Not to mention going on Google earth, finding my building on my acreage, zooming in on the roof, then on a front, side or rear elevation, then open the door and go inside the shop for a fly by all in 3D is pretty cool!

There is a shop layout utility on Grizzy's website.  You have to use the 2d outlines of their tools, tables and workbenches.  Pretty easy to give you an idea of infeed/outfeed space and general positions from various shop machines. 
http://www.grizzly.com/workshopplanner.aspx