Detect It Easy is a free portable tool for investigating the contents of files. It's designed as a packer identifier, something which tells you more about a Windows executable and how it was created, but it has other applications, too.
The program can identify more than 200 file types from their contents, for instance. If a file extension is missing or incorrect, drag and drop it onto Detect It Easy to find out more. Any supported file types will be displayed, and you'll sometimes get bonus information, too (the format version, like "1.4" for PDFs, or the number of files in an archive).
Need to know more? Click "H" and the file opens in a hex viewer. You're able to browse its contents at byte level, search for ANSI or Unicode text, dump a selected number of bytes to a file, and more.
If you're looking for a packer identifier then the program does a good job, as well as providing plenty of bonus information (it doesn't just calculate entropy but shows it graphically), and a disassembler for the real experts.
This is far more configurable and expandable than you might expect, too. For example, the logic to identify file types isn't hard-coded into the program, rather it's organised into scripts. You can view these within Detect It Easy, edit and even debug them.
Detect It Easy is an excellent tool for investigating files. The lack of documentation means it takes a while to figure out the program's more advanced features, but don't let that put you off - more basic tools, like the file identifier, don't require any knowledge or expertise at all.