SharePoint Online, as part of Office 365, is continually improved. The SharePoint Help Center is also continually updated with the latest changes. Below are some other resources on what's new.
The Office Blog covers updates to Office 365. Also, read how the updates enable new scenarios for others.
The SharePoint category filters the Office Blog to posts about SharePoint only.
Office 365 Roadmap
The Office 365 Roadmap is the official site for Office 365 updates and changes. In addition to listing updates available now, the roadmap also includes information about improvements that are coming soon.
Search for SharePoint to filter the roadmap.
For Admins Only
Administrators and IT Professionals should read Stay on top of Office 365 changes which covers update messages delivered through the Office 365 Admin Center.
Update Jan 23, 2018: This article was renamed from What's new in SharePoint Online and updated to reflect the official update sources.