Not sure if someone's already done this one, but I'm sure the variants I'm posting here haven't yet been done. Here's some relatively simple examples of Matrix-esque falling code, all in glorious full-screen!
The most Matrix-like, using a mish-mash of glyphs I chopped up from various fonts like the Katana font listed below.
<img src="http://dl.dropbox.com/u/41325628/construct2/matrix-rain/matrix-rain-screenshot.png" border="0">
Check out the demo here: Matrix Rain
And download it here: matrix-rain.capx
Katana Matrix Rain
Similar to the above, only relying on the Katakana font, and like the rest, uses Text objects rather than sprites.
Check out the demo here: Katana Matrix Rain
Download it here: katana-matrix-rain.capx
Binary Matrix Rain
Here's an interesting addition, a sort of precursor to a game project I'm currently working on, falling binary strings.
Check out the demo here: Binary Matrix Rain
Download it here: binary-matrix-rain.capx
Comic Sans Matrix Rain
Last but not least, made especially for all you Comic Sans Criminals out there, we have a Comic Sans variant! Using a colorful flurry of rotated characters, it's sure to be a favorite among graphic designers! ;)
Check out the demo here: Comic Sans Matrix Rain
Download it here: comic-sans-matrix-rain.capx