Streaming (8-bit) rot13 - still teaching myself streams, this seemed like a "useful" idea.

rot13.js

var through = require("through");
var rot13 = through(function write(buffer) {
    var transformed = new Buffer(buffer.length);
    var rot = 13;
    var lcase = { min: 97, max: 122 };
    var ucase = { min: 65, max: 90 };

    for (var i = 0, b = buffer.readUInt8(i); i < buffer.length; ++i, b = buffer.readUInt8(i, true)) {
        if (ucase.min <= b && b <= ucase.max) {
            b += rot;

            // Fix overflow
            if (ucase.max < b) {
                b = ucase.min + (b - ucase.max - 1);
            }
        } else if (lcase.min <= b && b <= lcase.max) {
            b += rot;

            // Fix overflow
            if (lcase.max < b) {
                b = lcase.min + (b - lcase.max - 1);
            }
        }

        transformed.writeUInt8(b, i);
    }

    if (this.paused) {
        this.queue(transformed);
    } else {
        this.emit("data", transformed);
    }
});

process.stdin.pipe(rot13).pipe(process.stdout);