My grandma needed help opening up Zip files so I was about to recommend Stuffit Expander.
Then I realized how much that is a bad piece of advice.
She would need to fill out a web form, respond to an e-mail, give her e-mail again and then download the file. After that she would get spam e-mails from them until the end of time.
Instead I recommended The Unarchiver. A simple download and I hope an easy solution to opening Zip files (and potentially others down the road).
Never again will I download Stuffit Expander based on the unending e-mails they are sending me.