Office Web Apps: And then there's Office Web Apps, Microsoft's free version of Office in the cloud that has been available since 2010. These web-based versions of Word, Excel, PowerPoint, and OneNote (plus Microsoft's new Outlook.com email service) are available through SkyDrive at http://skydrive.live.com. Office Web Apps have become increasingly capable, with features like commenting in Word, charting in Excel, transitions in PowerPoint, and ink recognition in OneNote.
Depending on your usage, Office Web Apps may be all you need. Since I can't find anywhere a comprehensive list comparing what's in Office 2013 or 365 versus Office Web Apps, the only way you can really tell if Office Web Apps has all the features you need is to try the service out.
If not, Office 365 is available as a preview, and you can also download a 60-day trial of Microsoft Office 2013 Professional Plus from Technet.