Google Docs & Spreadsheets has introduced a nifty context menu with options to convert and save Microsoft Office Word and Excel documents as Adobe PDF or OpenDocument .odt (for word processing documents) or .ods (for spreadsheets) formats. Word files can additionally be saved in an HTML, RTF or Text format.

Usage:

1) Upload a supported Word or Excel file to Google Docs & Spreadsheets
2) Right Click on the file in the Google File manager (on IE and Firefox only)
3) Select Save as Open Office or PDF for Excel files or Save as HTML, OpenDocument, PDF, RTF or Text for Word files.

The context menu also allows you to star/bookmark, hide or delete, add documents to different folders, share, preview or publish Word files publicly, create new folders or files online. You can also select a group of similiar format files and perform bulk operations on them.

Types of files that you can upload:

Documents (up to 500KB)
HTML files and plain text (.txt).
Microsoft Word (.doc), Rich Text (.rtf), OpenDocument Text (.odt) and StarOffice (.sxw).

Spreadsheets (up to 500KB)
Comma Separated Value (.csv).
Microsoft Excel (.xls) files and OpenDocument Spreadsheet (.ods).

Note: Unfortunately the context menu does not work in Opera 9.x. But if you would still like to use Google Docs and Spreadsheets for creating Word documents and Excel spreadsheets in Opera, check this article.