See what I'm doing? See it? That most basic option we've all so desperately wanted from Steam for as long as it's existed? Well, you can do it too, if you twiddle with the current beta.
You may well remember Alec found a neat third-party app that let you do this. Well now it can be done using just Steam itself.
It's a fiddle at first, but once done you're set. First of all, you'll need to be in the Steam beta. To get there you need to run Steam, click on the file menu option Steam, then pick Settings. In there, you'll see "CHANGE..." under "Beta participation". Click on that, and opt into the beta. It'll automatically restart, but when it's done you'll want to quit out completely.
Now you need to add a switch to the Steam shortcut. So find the icon, Start menu option or whatever shortcut you use to load it, right click on it, and choose Properties. In there you'll see the path it loads from looking something like this:
You want to add "-dev" to it, after a space, so it looks like this:
Run that, and now you'll see an extra page option, on the right after NEWS, called CONSOLE. Click on that one, and then type the following into the text field:
Hit return and it'll give you a window that looks like this:
Choose "Add install folder", and pick another place where you'd like to install Steam games. And you're done!
Now, whenever you attempt to install a game using the new file system (pretty much anything recent, and many will update), when you choose to install you'll have a dropdown box offering any of the folders you've set via the console option. It works. I tried it.
All props go to Reddit user RJacksonm1. And cheers to Craig for the tip.