I keep discovering keyboard shortcuts that are not listed anywhere under preferences. For example, I discovered here that ⌥ ALT⌘ CmdE will eject all partitions on a disk when a partition is selected. How the heck do people find these shortcuts, and where can I find more?
Not sure if this is a complete list, but this is a pretty good start from Apple:
Here is another link to an image of the shortcuts:
Quite a few keyboard shortcuts vary on different Macs, especially the laptops, so that is one good reason it is not easy to find a 100% complete list.
There is a super in-depth guide, here which was designed to show developers which shortcuts not to use, but seems to be the best go-to guide for the comprehensive ins-and-outs of keyboard shortcuts in OSX.
I'd also reccommend browsing around more on that same site (https://developer.apple.com), although I didn't see a definitive shortcut guide yet.
I've learned about many of the shortcut keys (including the ⌥-modifiers like the one you describe) by reading through the Apple Pro Training Series books, by Peachpit Press.