They can all be used stand-alone - but carefully. In stand-alone mode, there is no File menu for Open or Save As. So, a copy of your image should be made first for safety. Then, right-click the image and Open With > [Nik tool] - adjust as desired and click Save, replacing the image file.
Hosting is best (layers). But standalone is fine for folks on a budget (students).
I found this blog that gives some more details for people without Photoshop, Lightroom, etc:
http://edknepleyphoto.com/2013/06/15/using-the-nik-collection-in-a-stand-alone-mode/
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