Taking file1.js and file2.js and combined them together to make file3.js
Using things like JsMin or YUICompressor
URL has /scripts/file2.js?v=123456
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EDIT: I work primarily in SharePoint Designer, so my methods may not match yours. I'm more likely to do this to add my scripts to pages, whether the master page, page layouts, or individual aspx pages.
Yes, I would always use the minified versions in a "production" environment (where I didn't want to do any debugging).
The ?v= thing is sometimes required if you have overzealous caching going on in your environment. If you use unique names for your script files, then it's probably not necessary.