2. Firefox or Chrome
Assuming you have Windows, you probably have Internet Explorer as well. There are many reasons to consider installing something else, however--either in addition or instead of that browser--and Firefox and Chrome are the most obvious examples.
Chrome recently assumed the No. 2 spot in the browser market--displacing Mozilla Firefox--and it's widely considered the most secure choice. Firefox, however, just got a really speedy update, and it's designed with numerous ethical considerations in mind.
All in all you really can't go wrong with either of those, and of course they're both free.
Also made by Mozilla, Thunderbird is free, cross-platform e-mail client software that I've been using for years. It's very easy to set up and customize, and it's available in countless languages. Numerous extensions are available as well.
It used to be OpenOffice.org that was the most commonly recommended free and open source office productivity suite, but over the past year or so LibreOffice has taken its place.
I use LibreOffice every day for my writing, and I can attest that it's a great alternative to Microsoft Office and the other big competitors out there. Just like Office, it includes modules for word processing, spreadsheets, and presentations.