Google Chrome (Safari on macOS is also very good!)
Google Earth – see the world
CCleaner – remove unnecessary files on Mac hard drive
Skype – communicate with audio/video with other devices
Teamviewer – remotely control other devices
Malwarebytes – scan and remove unwanted malware
VLC – VideoLan player – play most video / audio file formats
Dropbox – store and share files in the cloud
In the Mac App Store:
Mactracker – great comprehensive list of Apple hardware over the years
Coconut Battery – check internal battery usage/status on Mac and iOS devices
Memory Clean 2 – free up unused RAM on Mac