Free office365 for developers
You can sign up for a Microsoft Office 365 developer account free for 1 year, which gives you access to full Microsoft Office (including app downloads) for up to 25 users, with 5 app installs per user, Skype for Business, Sharepoint and Exchange email.
Obviously if you want to continue after a year there is a cost.
I posted this deal this morning and deleted it as you'll find there's no keys left.