I have a 27" iMac and find that I need to use one resolution to read and another to edit photographs. This iMac's native display (2560 x 1440) presents text that is just too small to ready easily with my near-sighted eyes. I've always use Apple's Displays System Preference to switch resolution when needed. In my previous system, Snow Leopard (OS X 10.6.8), I enabled the Displays menu extra and clicked the icon in my menu bar to access the preference, but that functionality was inexplicably removed in Mountain Lion (OS X 10.8).
QuickRes is a welcome solution to your resolution woes. This screen resolution switching app from ThinkDev (formerly Inertiactive) gives you back what Apple removed. Recently upgraded to version 3.1, QuickRes allows you to switch between a variety of resolutions for your Macintosh. On a MacBook with Retina display, it lets you take your screen resolution all the way up to 3840 x 2400, but alas, I could not test this feature.
After you install QuickRes a simple dialog appears with a few basic instructions.
You can choose your preferred keyboard shortcut using three to four keys. I left mine at the default of Command-Option-R.
If you have a non-Retina display Macintosh you can Enable HiDPI Mode, which according to ThinkDev, "is as close as you can get to a Retina display on a standard display." A warning dialog, which may be ThinkDev humor, popped up when I clicked to use HiDPI mode. As, AppleCare just replaced my newest iMac because of a graphics display problem, I opted not to test this feature. One replaced iMac per decade is all I can stand, sorry.
The QuickRes Preferences include four tabs to let you show notifications when the resolution changes, show the QuickRes icon in your menu bar, and indicate the number of the resolution you chose. Your chosen resolutions show up in a numbered list in the Resolutions tab in the preferences. You can choose up to eight different resolutions in which to set your display in the Advanced tab. I found that two is sufficient, but it depends on how you work and what software you work within.
You can use your mouse to scroll through your chosen resolutions with a simple click on the menu bar icon. A right-click (also called a control-click) lets choose a new resolution from the pop-up list. Alternatively, you can use your chosen keyboard shortcut.
One note of caution though, do not quickly switch resolutions back and forth. That seemed to yield a stuttering complaint from my graphics card. QuickRes also refused to work at all a couple of times after I switched to a different resolution too quickly.
I found it a little annoying that when I changed resolutions the cursor jumped to some random location on the screen. Moving the mouse revealed its new location, but I am not sure why the cursor doesn't stay in a near place to where it started. I did not experience any scaling problems when switching between resolutions, as some readers mentioned on the App Store page. Once, after a reboot, the program no longer worked and required a reinstall, but I don't think this is a major bug, just a little glitch.
The Advanced tab in the Preferences gives you the option to Open QuickRes on Login. This seems to be the only way to make sure the app still works after a reboot and it works reliably.
Mouse jumping aside, the program is easy to use and set up. It solves the problem of needing to open the System Preferences to perform a simple task and it gives you back the ease of use experience you should have on your Macintosh.
The free QuickRes version 3 can be downloaded from the App Store, but it only lets you switch to 2880 x 1800, but not switch back. You can buy the US$1.99 version 3.1 QuickRes on ThinkDev's site. TUAW readers can get the updated version at half price from the link below. QuickRes works in OS X 10.8 and has also been tested to work in OS X Mavericks (OS X 10.9).