Do you know how many file types Chrome, Firefox, Safari et al can handle? It's probably more than you think, and if you need to quickly view a file—be it a PDF, MP3 or photo—then you can use your Web browser for the job. You don't have to wait for iTunes or Photoshop to kick into action, and you can simply close the browser tab when you're done.
The easiest way to do this is to simply drag the file in question from your system to an open browser window. You can also use the Open With command (right-click a file in either Windows and Mac OS X). To change the settings for a file type permanently, use the Choose Default Program option on the Open With menu in Windows or the Change All button on the Get Info dialog on a Mac.
Alternatively, you can type a file path address directly into the address bar of your browser and navigate to a file that way—use "C:\" on a Windows box or "file:///Users/" on a Mac. Hitting Ctrl+O or Cmd+O, meanwhile, brings up the open file dialog, which achieves the same purpose. You should be okay with plain text files, PDFs, and basic audio, video and image formats.
It's a simple trick that might save you some time—or at least keep you streamlined.