Are there any free or open source .IGS viewers that you've actually used on a regular basis? There are a lot of Google hits, but they seem like they're all free trials, misdirection to other products, etc.
After some googling around I found IGSViewer, it's for Windows and should be free.
After a quick test it didn't work for the specific IGS file we were trying to view: there was just the outline but many internal details were missing. Don't know if the problem lies in the file or in the viewer, didn't try any harder (we found a way to get the source, a DXF drawing from which someone also made the IGS one, and used a dxf viewer that worked really well)