Viewers

Sooner or later, you'll need to look at a file and you won't have the right program installed. Rather than buy a new package just to view it, you can probably find a viewer that will solve your problem.

This list covers most of the file types I've run into recently. I'll update and add as appropriate.

Acrobat
http://www.adobe.com/products/acrobat/readstep2.html

AutoCAD
VoloView has been replaced by Autodesk DWF Viewer
http://usa.autodesk.com/adsk/servlet/index?id=2787358&siteID=123112

IrfanView
Supports most common graphic formats, creates slideshows, basic editing tools
http://www.irfanview.com

PowerPoint 2003
http://www.microsoft.com/downloads/details.aspx?
FamilyID=428d5727-43ab-4f24-90b7-a94784af71a4&displaylang=en

ViewCompanion
http://www.softwarecompanions.com/viewcomp.html

Visio Viewer 2002
http://www.microsoft.com/downloads/details.aspx?
FamilyID=8fad9237-c0a7-4b80-a5df-46ce54dad2df&displaylang=en

XnView
Most common graphic formats including AutoCAD
http://www.xnview.com