Keyboard Shortcut For Entering Screen Saver
If you open Keychain Access (I use Spotlight, but you can go to /Applications/Utilities), go to the main menu and hit Preferences... (or Command-,) you'll find a checkbox labeled "Show keychain status in menu bar..." If you check it, you'll get a little lock in your menubar.



If you click "Lock Screen", your screensaver will activate. I use this at work as an OSX Control-Alt-Delete alternative.

(Note: it will require your user password to unlock the screen. This may/may not be desirable.)