Interact with OS X screensavers
Pressing any key on the keyboard may not turn off your screensaver, and could instead allow you to control it a little.
Apple includes a number of screensavers in OS X that provide you with options to exercise your display and prevent LCD ghosting, CRT burn-in (if you still use a CRT monitor), or other persistent images from being a nuisance. As with most screen savers, you can deactivate them by moving your mouse or pressing a button on your keyboard; however, this may not always be the case, and depending on the screensaver being used you may instead find the screensaver will persist even when some inputs are pressed.
Advance photos in slideshows
Apple includes a number of slideshow screensavers that will present built-in images or those from various photo libraries on the system. With these slideshows active, if you press the left or right arrow keys, then the system will advance through its choice of photos being used for the slideshow. Unfortunately in doing so it will avoid the transition effects for the slideshow; however, it will allow you to in a way peruse through pictures in your library.
Change albums in iTunes artwork
Another popular screensaver is the iTunes artwork saver, which will show a grid layout of the albums in your iTunes library. With this screensaver, moving the mouse will not deactivate it but instead allow your cursor to hover over an album. In doing so you can then click the album to play or pause songs from it.
Function keys still work
With all screensavers, you should be able to use the Apple-supplied Function key commands without deactivating the screensaver, which include adjusting the screen brightness, keyboard backlight, volume, and the music playback controls. Do keep in mind that for these to work they will have to be the default function for these keys. Apple supports switching these keys to be regular F-keys in the Keyboard system preferences; however, with this setup they cannot be accessed if the screensaver is active.
Tab does nothing
A last key that will not disable the screensaver is the Tab key. Unfortunately this key also does not provide any input to the screensaver, so while its lack of input may simply be a bug, currently pressing it will simply do nothing.