Here are the steps to open a .rar file without requiring any software.
1. If you have got the file from an email, send it to your gmail account.
2. Do not download the file. But instead, click on the view button. (to view it as google docs.) 3. Now, in the google docs, you can see that the file is unzipped itself and the original file is available for download.
So, hit the download button and the unzipped original file will download to your computer.
Hope it helps.