Chrome: If you're stuck on a machine without Microsoft Office and you get an Excel spreadsheet or PowerPoint presentation, you don't have to go through hoops uploading it to another service and hoping the translation goes well. Chrome Office Viewer views Word, Excel and PowerPoint documents inside of Chrome flawlessly.
Chrome Office Viewer is fast, free and loads Office documents without having to install additional software just to get the fonts or formatting to display properly. Plus, it avoids the need to upload your document to Google Docs (for example) just to look at it. Of course, it's a document viewer, so if you want to edit the file, you'll need to find another tool, but if you just want to look at or print an attachment someone sent you, this extension does the trick.
Chrome Office Viewer (Free) [Chrome Web Store]