First, the bad news: When I'm using even a relatively simple application like TextEdit, there are over 75 possible keyboard shortcuts I could press. I'm including here not just menu commands belonging to TextEdit itself, but shortcuts inserted into the Services menu by applications scattered all over my hard disk, plus system-wide global keyboard shortcuts enabled by the system and by various preference panes and faceless background applications.
Who on earth can memorize 75 keyboard shortcuts? How is anyone supposed to deal with this? And of course, things change radically each time I switch from one application to another. Some applications have literally hundreds of keyboard shortcuts! As a result, I find myself resorting to menus much more than to keyboard shortcuts; and even then, I'm frequently confused when I do try a keyboard shortcut and some utterly unexpected, surprising thing happens (typically because I got it wrong, and some service or global shortcut responded).
Okay, now the good news: Ergonis Software's now knows about most of these keyboard shortcuts. You remember KeyCue, right? I reviewed it when it first came out (see " ", 2004-06-04). KeyCue lets me specify a modifier-key combination which, when held down for a while, causes a reminder list of currently available keyboard shortcuts to appear, without my having to hunt through any menus. Then I can proceed to choose one. Not only do I rapidly acquire a muscle-memory for those keyboard shortcuts I use most, but also I learn about keyboard shortcuts I probably wouldn't even have discovered otherwise.
In that original review, I wrote: "What I really want is a cheat sheet that lets me see and choose from all of an application's menu items, whether or not they have shortcuts, and I'd also like a cheat sheet showing all the global hot keys that various applications have installed." Ergonis has now responded to the second wish on that list. KeyCue 4.4 knows about at least some global hotkeys. I say "some" because it is easy to find global hotkeys of which KeyCue knows nothing. For example, on my machine, Command-Control-Option-T toggles on and off, and Command-Control-Option-W summons the  window list; but KeyCue's cheat sheet doesn't list those. Still, the improvement in this version, together with the previous version which introduced user customization of the modifier key-combination that activates the KeyCue window, is palpable and very welcome.
A single-machine KeyCue license is 19.99 euros. It requires Mac OS X 10.4 Tiger or later, and there's a 1.1 MB trial.