If you need to prepare drawings for a patent application, you have two major choices: whether to use a drafting company or a software program. I did an unscientific survey on the Oppendahl EFS-Patent Listserve (http://www.oppedahl.com/listserves.html), and here is what people are using:
OmniGraffle https://www.omnigroup.com/omnigraffle/
DraftSight http://www.3ds.com/products-services/draftsight-cad-software/
RFFlow http://www.rff.com
FreeCad http://www.freecadweb.org Free and open source CAD package; said to be a bit buggy and has a fairly steep learning curve, but supports a large number of CAD formats and can convert esoteric formats to something more usable in other programs on this list.
Gimp2 http://www.gimp.org Free and open source photoshop alternative, said to be good for tracing client-provided cad images and touching up images to make line drawings, among other things
Inkscape https://inkscape.org/en/ Free and open source vector graphics program, good detection/conversion algorithms to make bitmaps editable, often useful in conjunction with Gimp
IrFanView http://www.irfanview.com Useful for making sure the final image is really pure black and white. There is no native-Mac version of IrfanView. However, you can use IrfanView in conjunction with Mac program like WineBottler/DARWINE.
PDFSam http://www.pdfsam.org Free, open source platform independent, useful for splitting and merging PDFs
Adobe Illustrator http://www.adobe.com/products/illustrator.html (you may be able to get it as part of a Creative Cloud package)
LibreOffice Draw comes highly recommended. http://www.libreoffice.org/ It addition to LibreOffice itself being a Microsoft Office replacement (note that LO includes Math, a formula editor) it includes an extremely versatile viewer. LibreOffice Draw is cross platform and can edit Visio files as well as a host of other formats. Free, open source. It’s very powerful, with all of the tools of PowerPoint and many of those of Visio. Draw allows merging/subtracting shapes to produce complicated polygons and more. It's not a 3D CAD program, but I have seen pretty good drawings from it.
PowerPoint has a viable drawing program, but you may find it tiresome for more than simple flow charts and block drawings. You may already own this as part of Microsoft Office, either PC or Mac. For that matter maybe there is a Keynote expert out there who is using that program?
If you are using Solidworks (or if a client sends you Solidworks files) then check out eDrawings viewer. http://www.edrawingsviewer.com eDrawings only works with Solidworks files, but is free, simple to use, polished and allows you to manipulate 3d models and generate wireframe models with solid surfaces of any view you like. Check out the presets, which work well for fully defining a design for use in design patents. If you like that program and want to convert models to something that will work with eDrawings viewer check FreeCAD (listed above). It was pointed out to me that eDrawings Viewer allows viewing of AutoCAD DWG and DXF files in addition native SOLIDWORKS parts, assemblies, and drawings. Further, if clients save/output or convert the native CAD file(s) from their CAD software to any of the following eDrawings file formats (edrw, eprt, easm, edrwx, eprtx, and easmx), the eDrawings viewer also supports these file formats. I hope this makes the sometimes challenging job of working with client supplied CAD files a bit easier.
Also mentioned were Microsoft Word and Microsoft Excel, only as legacy programs.
If you are on a PC, then Microsoft Visio is an option. Another PC option is smartdraw. Also, any number of other CAD programs probably will work. If you have a favorite, drop me a note, and I will mention it here.
While no one mentioned this, my experience shows that you never can tell what program an inventor uses, and what he or she might send you. You don’t want to appear in the dark ("Hey, I can’t open this file!”) and nothing instills confidence like a library of file viewers (“Just send me whatever you have, I can probably open it.”)
Thanks to all who participated! Most patent practioners who answered are doing flowcharts and circuit diagrams themselves. Drawings with complex figures are sent to professional drafters.
If you use a drafting company, be alert to confidentiality issues, and location of the drafter. Several people noted the following notice from the USPTO:
http://www.uspto.gov/web/patents/patog/week53/OG/TOCCN/item-115.htm
Before you use a drafting company that may be offshore, be sure to read that notice.
And, if you do use a draftsperson, ask for a file in a format that can be tweaked at your end for last minute changes.