node.js

var sys = require('sys')
    filename = process.ARGV[2],
    spawn = require('child_process').spawn;

if (!filename)
    return sys.puts("Usage: node watcher.js filename");

// Look at http://nodejs.org/api.html#_child_processes for detail.
var tail = spawn("tail", ["-f", filename]);
sys.puts("start tailing");

tail.stdout.addListener("data", function (data) {
    sys.puts(data);
});

// From nodejs.org/jsconf.pdf slide 56
var http = require("http");

http.createServer(function (request, response) {
    response.writeHead(200, {'Content-Type': 'text/plain'});
    tail.stdout.addListener("data", function (data) {
        response.write(data);
    });
    //response.end('Hello World\n');
}).listen(8124);

root@core:~# echo 'line 10' >> foo