When it comes to undocumented system tweaks, we at TidBITS tend to take a fairly conservative stance. After all, your system is a system. It’s responsible for running your whole computer. You wouldn’t want to break it accidentally, and we wouldn’t want to give you any advice that might cause you to do so. Also, undocumented tweaks are undocumented; this means that Apple could withdraw their effectiveness at any time (and has indeed sometimes done so; see, for example, “,” 29 September 2008).
Sometimes, however, Apple backs us into a corner, producing a system that does something so blatantly annoying or even downright moronic that we can’t resist advising you to fix it by giving some mystical and unsupported incantation at the command line. For example, when Mac OS X 10.5 Leopard introduced the transparent menu bar, I couldn’t get any work done, and rejoiced the moment a trick was discovered for making it opaque again (“,” 16 November 2007). Apple later saw the error of its own ways (for once!) and provided an official interface for doing the same thing, which remains to this day. And when 10.7 Lion deviously hid your user Library from you, Adam immediately told you how to show it again (“ ,” 20 July 2011).
When it comes to undocumented system tweaks, what most users want, I think, is two things:
Give me a graphical interface. Don’t make me type directly into Terminal; I’m afraid I might mess something up accidentally. And if I don’t like a change I’ve made, I want a simple way to undo it immediately.
Give me a conservative list. I don’t want to go wild and hack my system; I just want to know what’s well-tested and safe that I can tweak, even though Apple doesn’t provide an interface in System Preferences to let me do so.
If that’s how you feel, you can’t do better than to download Marcel Bresink’s freeware. TinkerTool is a brilliant one-window application that presents itself as a series of panes, rather like System Preferences, each pane providing checkboxes or other interface for toggling undocumented under-the-hood switches in your system and in some Apple-provided software, such as the Finder and Safari. It customizes itself automatically to the particular system and version you’re using, so the options you’ll see are always the right options. And, like TidBITS, it takes a conservative stance; its only options are those that have been determined to be safe and useful.
You can get a sense of what TinkerTool might be able to do for you by looking at the online in 10.6 Snow Leopard and 10.7 Lion. For example, I always check the option listed here as “Disable the three-dimensional glass effect of the Dock”; this allows me to keep the Dock at the bottom of the screen while using the more compact, pleasing appearance that it automatically takes on when it’s at one side of the screen. In Snow Leopard and before, I always checked “Place both scroll arrow buttons of any scroll bar at both ends of the bar,” giving me both an up arrow and a down arrow at each end of the scroll bar. (In Lion, there are no scroll arrow buttons in a scroll bar, so that option no longer applies.) And the option listed as “Control the style and degree of font smoothing (optimized for CRT or LCD)” has saved my eyes through many system generations; because of a bug in the system, my LCD monitor is not seen as an LCD, so that Apple’s own system preference panes (Appearance or General) don’t allow me to increase the font smoothing far enough to make text legible — whereas TinkerTool does.
For Lion in particular, I’d call your attention to several settings you might consider:
Lion has a feature, copied from iOS, where holding down a letter on your keyboard can summon a popover listing alternate versions of that letter, from which you can then choose. For example, the popover that appears when you hold down “e” includes “e” with an acute accent, “e” with a grave accent, and so forth. The price of this feature, however, is that you can no longer hold “e” to achieve multiple repeated “eeeeee”, as you could with previous systems. Users writing horror stories might object to that. TinkerTool lets you access the setting that restores the old behavior (“Re-enable the key repeat feature”); you will then lose the popover, but of course you can still type alternate letter forms just as before, using the Character Viewer, the Keyboard Viewer, or a modifier-key combination or sequence.
When you close a window containing unsaved changes and the dialog appears asking whether you want to save, you might have a muscle memory telling you that you can dismiss the dialog without saving by pressing Command-D, for “Don’t Save.” In Lion, that keyboard shortcut no longer works; you can restore it with the TinkerTool option listed as “Re-enable the keyboard shortcut for Don’t Save in save sheets”. (Of course, that little dialog about unsaved changes itself appears less than it used to, because of Lion’s Auto Save feature. But that’s a horror story of a different kind.)
Most Lion users are aware by now that when an application is launched, whatever windows were open when that application was previously quit will automatically re-open, thanks to Resume. If you don’t like this, you can turn off Resume at a global level; it is also possible, even if Resume is globally turned on, to turn off Resume for a particular application on a one-time basis, by holding Option as you quit it (or vice versa). But what if you’d like, say, to turn off Resume globally but turn it on by default for certain particular applications? It turns out that there is a way to do this, but Apple doesn’t provide access to it; TinkerTool does (“Control the Resume setting individually per application”).
TinkerTool is not the only system-tweaking game in town. There are other applications that do the same sort of thing TinkerTool does. For certain settings, they might provide a better interface; for example, in the case of the per-application Resume setting, I rather prefer Erica Sadun’s Glenn Fleishman in Macworld) and get rid of the iCal and Address Book leather appearance (though I’m not sure I can recommend that one, as it involves meddling with the internals of these applications).. For other settings, they might provide graphical access to something that TinkerTool doesn’t; for example,  lets you enable AirDrop on unsupported hardware (providing a graphical interface to the trick discussed by
Nevertheless, I always find myself returning to TinkerTool as my under-the-hood control panel of choice. It’s a utility that I’ve used since the early days of Mac OS X, and I recommend that you try it out if you haven’t already. You might be surprised at some of the simple ways in which it can make your Mac more usable, more comfortable, or more powerful.
TinkerTool is a 1.5 MB; the current version requires Snow Leopard or Lion, but earlier versions have been spun off into  (TinkerTool Classic and TinkerTool Classic Generation 2), still available to those using older systems.