Microsoft's new Office Web Apps will let you work between your desktop Office documents and the web, but it's currently in an invite-only preview - and the invites are long gone. However, it turns out that all you need to do is log into Microsoft's SkyDrive using a Live.com account (if you don't already have one, registering is free) and then upload any Microsoft Word doc. When the upload completes, SkyDrive will ask you if you want to join the Office Web Apps preview program .
Once you've joined, you'll be able to view any uploaded Word doc, Excel spreadsheet, or PowerPoint presentation from your browser in Microsoft's (currently barebones) online offerings. Word documents are currently read-only, but Excel docs and PowerPoint presentations are fully editable.