So I've been playing a lot of Minecraft lately.  Since most of the instructional/FAQ type material is teenagers making Youtube videos, here's the wisdom of those videos in condensed form.

• In Linux, most of the game files live in a hidden folder in your home directory called ".minecraft".  Ditto for OS X.  I haven't played it on Windows, so I can't say what the equivalent is there.
• If you want to install a texturepack, go to .minecraft/texturepacks and copy in the zip file you downloaded.  No need to unzip it.  The texturepack will be named the same as the zip file, generally.
• A lot of the sites with texture packs, mods, etc, are super shady.  For messing around with my save files, I finally settled on a python app called "MCPlayerEdit".  It's on github rather than a shady download site, and works well.
• The only map viewer program I've tried was Minecraft Overviewer.  The default install didn't work because I had Python 2.7, so if you're a Mac/Linux user, just go ahead and compile it yourself and it'll work fine.