matrix.js allows easy creation of an animated matrix-style background with JavaScript and CSS.

matrix.css

div.matrix {
    color: #060;
    position: absolute;
    font-family: mono-space, sans-serif;
    font-size: 10px;
    top: 5px;
    left: 10px;
    z-index: 2;
    width: 99%;
    overflow: hidden;
}

matrix.js

$(document).ready(function(){
    var numLines = document.documentElement.clientHeight / 10;
    var numChars = document.documentElement.clientWidth / 5;
    for(var n=0;n<numLines;n++) {
        var line = "<div class=\"matrix\" style=\"top:"+(5+n*10)+"px\">";
        for(var i=0;i<numChars;i++) line += "<span id=\"matrix_"+n+"_"+i+"\">"+Math.round(Math.random())+"</span>";
        line += "</div>";
        $("body").prepend(line);
    }
    
    var last_pos = [];
    setInterval(function(){
        var row = Math.round(Math.random()*(numLines-1));
        var col = Math.round(Math.random()*(numChars-1));
        if(last_pos.length >= 50) {
            $("#matrix_"+last_pos[0].row+"_"+last_pos[0].col)
                .css('color','#060');
            last_pos = last_pos.slice(1);
        }
        $("#matrix_"+row+"_"+col)
            .text(Math.round(Math.random()))
            .css('color','#0f0');
        last_pos.push({'row':row, 'col' :col});
    }, 100);
});