"Free" isn't going to happen unfortunately - lighting visualisation software is extremely difficult to write and thus the packages tend to be fairly expensive.
The two main 'market leaders' in this field at present are CAST's "WYSIWYG", and Capture Sweden's "Capture 2005".
Both of these products include simulations of the majority of popular lighting fixtures, so you wouldn't need to download anything else. They also can be used live with a lighting console.
They behave differently and have different primary strengths - Capture (currently) has a much better live visualisation, while Wyg can render better still images.
Both these products are available in a freely downloadable demo format - I believe that neither of them permit you to save in the demo version.
I quite like Capture, but I know many people who love WYSIWYG.
I'd suggest downloading the demos of both packages and seeing if either will meet your needs.
Whilst I tend to use a cocktail napkin and a Sharpie, there are many others who prefer a more refined approach.
I would suggest an Amazon search for "Lighting Design" - you will find many titles that cover the design and documentation stage in detail.
www.etcconnect.com