Can I run top as screensaver?

If you're looking for hackerish screensavers, the old Xscreensaver bundle was converted to OSX a while ago.

Nicely hacker type examples would be Sonar, AppleII, and Phosphor. Possible GLMatrix too.

However, to answer your question: https://superuser.com/questions/420546/osx-run-bash-or-app-as-the-screensaver

Write an applescript to run Terminal, run Top, and fullscreen itself. Then put that script into the app.

If you install the OS X version of XScreenSaver, the phosphor saver simulates an old phosphorous terminal display. It can be configured to run any command in the terminal display, including top:

I find the settings -scale 2 -delay 40 -program top work quite well, and look quite "hackerish".

The LookThrough screensaver does nearly what you are looking for on Mavericks and Yosemite. It permits to maintain your Mac correctly secured when at the same time you maintain on screen the actual output of a top command.

The output on screen won't be exactly a synchronous copy of what would be on screen without the screen saver. This output will be an asynchronous copy, but the delay is so small that it is perfect to avoid completly eating the CPU of your Mac and to avoid to give you the feeling to be on slow motion.

It does also let you maintain on screen whatever you want. I use it to keep on screen graphs of cacti output of network equipment performance.

If you are looking for a solution on Lion or Mountain Lion, please look at this old question Transparent locked screen.

You can use ScriptSaver, since, if you have Require password after sleep or screen saver begins enabled after IIRC OS X 10.7 you'll otherwise not see through and get a black screen (I'm on 10.8.5). Here's an example of using it to launch iTerm with top.

Activation Script

tell application "iTerm" to create window with profile "Hotkey Window" command "top"

Screen Saver

LookThrough. Alternatively, JohnnyNash, Transparent.

Deactivation Script

tell application "iTerm"
close front window
quit
end tell