Often, I will attach PowerPoint presentations to my calendar so students who are absent can view what we did in class and take notes.  This is great--IF you have PowerPoint on your computer.  But if you don't, how can these presentations be viewed?
There are two options--two free options at least:
1.  Download PowerPoint Viewer from Microsoft. This will allow you to view any presentation I download to the calendar.
2.  Download Open Office--a free word processing program that includes a presentation program. It will open and convert my presentations. There may be slide changes--fonts may differ, animations may or may not run--but it's better than a sharp stick in the eye.