I'd really reccomend the free Marvin suite from ChemAxon.
MarvinSketch (the actual drawing package) hasn't disappointed me in the past and has a wide feature set. In addition to just being able to draw compounds in Lewis format, the software includes plugins to name what you have drawn (systematic/traditional), predict properties, change atom and bond properties, generate stereoisomers and much more that I haven't got the experience to do anything more than experiment with!
You can view what you have drawn in Lewis form and also as a 3D structure should you wish to. It supports many chemical drawing filetypes.
Here is the product page, including brochure and technical details. It is Java based so works on Mac, Windows, Linux and is even available on the web (but is quite slow to generate properties etc. - I'd reccomend doing this locally).