A welcome trend that I’ve noticed in the last one year or so is that many binary/proprietary file formats are often using a more universal file format under the hood.
For example, Firefox extensions are packaged as .xpi files. But really, these are just zip files. You can rename the file with a .zip extension and check out all the files in an extension very easily.
The other file formats that I know of are:
Office 2007 formats (.docx,.xlsx,.pptx etc.) – Zip files
NZB files (Usenet Downloads) – XML files
EML (Email message files) – TXT files
Got any others that you are aware of? Share it in the comments!