Matrix Screensaver is a small and simple app which displays the raining-green-text effect from The Matrix.
It's an open-source product, and just about as stripped back as it could be. The entire download is only 131KB, and that's still enough to squeeze in both 32 and 64-bit editions.
There's no installation required, and no settings or options. Simply double-click the .SCR file to see the effect, and press a key or move the mouse to make it disappear.
If you're happy with the results, right-click the SCR file and click Install. The regular Screen Saver applet appears, and you can set the idle time before you'd like the program to kick in.
Type screensaver in the Windows Search box to return to the applet later, or enter control desk.cpl,screensaver,@screensaver in the Win+R run box.
Matrix Screensaver is extremely basic, just a single raining-green-text effect which you can't tweak in any other way.
On the plus side, it looks reasonable, is tiny and doesn't have any adware or other unwanted extras. Take a look.