Ivo Wetzel's answer doesn't seem to be valid in Socket.io 0.9 anymore.

In short you must now save the socket.id and use io.sockets.socket(savedSocketId).emit(...) to send messages to it.

This is how I got this working in clustered Node.js server:

First you need to set Redis store as the store so that messages can go cross processes:

var express = require("express");
var redis = require("redis");
var sio = require("socket.io");

var client = redis.createClient()
var app = express.createServer();
var io = sio.listen(app);

io.set("store", new sio.RedisStore);


// In this example we have one master client socket 
// that receives messages from others.

io.sockets.on('connection', function(socket) {

  // Promote this socket as master
  socket.on("I'm the master", function() {

    // Save the socket id to Redis so that all processes can access it.
    client.set("mastersocket", socket.id, function(err) {
      if (err) throw err;
      console.log("Master socket is now" + socket.id);
    });
  });

  socket.on("message to master", function(msg) {

    // Fetch the socket id from Redis
    client.get("mastersocket", function(err, socketId) {
      if (err) throw err;
      io.sockets.socket(socketId).emit(msg);
    });
  });

});

I omitted the clustering code here, because it makes this more cluttered, but it's trivial to add. Just add everything to the worker code. More docs here http://nodejs.org/api/cluster.html

Well you have to grab the client for that (surprise), you can either go the simple way:

var io = io.listen(server);
io.clients[sessionID].send()

Which may break, I hardly doubt it, but it's always a possibility that io.clients might get changed, so use the above with caution

Or you keep track of the clients yourself, therefore you add them to your own clients object in the connection listener and remove them in the disconnect listener.

I would use the latter one, since depending on your application you might want to have more state on the for the clients anyway, so something like clients[id] = {conn: clientConnect, data: {...}} might do the job.

A simpler solution than what was proposed so far, tested with socket.io v1.0.4:

Server

server.js

var
    io = require('socket.io'),
    ioServer = io.listen(8000),
    sequence = 1;
    clients = [];

// Event fired every time a new client connects:
ioServer.on('connection', function(socket) {
    console.info('New client connected (id=' + socket.id + ').');
    clients.push(socket);

    // When socket disconnects, remove it from the list:
    socket.on('disconnect', function() {
        var index = clients.indexOf(socket);
        if (index != -1) {
            clients.splice(index, 1);
            console.info('Client gone (id=' + socket.id + ').');
        }
    });
});

// Every 1 second, sends a message to a random client:
setInterval(function() {
    var randomClient;
    if (clients.length > 0) {
        randomClient = Math.floor(Math.random() * clients.length);
        clients[randomClient].emit('foo', sequence++);
    }
}, 1000);

Client

client.js

var
    io = require('socket.io-client'),
    ioClient = io.connect('http://localhost:8000');

ioClient.on('foo', function(msg) {
    console.info(msg);
});

How to run

Install the required libraries:

npm install socket.io
npm install socket.io-client

Run the server:

node server

Run every client in a separate console:

node client

In 1.0 you should use:

io.sockets.connected[socketid].emit();

each socket joins a room with a socket id for a name, so you can just

io.to(socket#id).emit('hey')

docs: http://socket.io/docs/rooms-and-namespaces/#default-room

Cheers

io.sockets.sockets[socket.id].emit(...) worked for me in v0.9

Socket.IO allows you to “namespace” your sockets, which essentially means assigning different endpoints or paths.

This might help: http://socket.io/docs/rooms-and-namespaces/

Whatever version we are using if we just console.log() the "io" object that we use in our server side nodejs code, [e.g. io.on('connection', function(socket) {...});], we can see that "io" is just an json object and there are many child objects where the socket id and socket objects are stored.

I am using socket.io version 1.3.5, btw.

If we look in the io object, it contains,

 sockets:
  { name: '/',
    server: [Circular],
    sockets: [ [Object], [Object] ],
    connected:
     { B5AC9w0sYmOGWe4fAAAA: [Object],
       'hWzf97fmU-TIwwzWAAAB': [Object] },

here we can see the socketids "B5AC9w0sYmOGWe4fAAAA" etc. So, we can do,

io.sockets.connected[socketid].emit();

Again, on further inspection we can see segments like,

 eio:
  { clients:
     { B5AC9w0sYmOGWe4fAAAA: [Object],
       'hWzf97fmU-TIwwzWAAAB': [Object] },

So, we can retrieve a socket from here by doing

io.eio.clients[socketid].emit();

Also, under engine we have,

engine:
 { clients:
    { B5AC9w0sYmOGWe4fAAAA: [Object],
      'hWzf97fmU-TIwwzWAAAB': [Object] },

So, we can also write,

io.engine.clients[socketid].emit();

So, I guess we can achieve our goal in any of the 3 ways I listed above,

1. io.sockets.connected[socketid].emit(); OR
2. io.eio.clients[socketid].emit(); OR
3. io.engine.clients[socketid].emit();

You can do this

On Server Site

global.io=require("socket.io")(server);

io.on("connection",function(client){
    console.log("client is ",client.id);
    //This is handle by current connected client 
    client.emit('messages',{hello:'world'})
    //This is handle by every client
    io.sockets.emit("data",{data:"This is handle by every client"})
    app1.saveSession(client.id)

    client.on("disconnect",function(){
        app1.deleteSession(client.id)
        console.log("client disconnected",client.id);
    })

})

    //And this is handle by particular client 
    var socketId=req.query.id
    if(io.sockets.connected[socketId]!=null) {
        io.sockets.connected[socketId].emit('particular User', {data: "Event response by particular user "});
    }

And on client site it is very easy to handle

var socket=io.connect("http://localhost:8080/")
    socket.on("messages",function(data){
        console.log("message is ",data);
        //alert(data)
    })
    socket.on("data",function(data){
        console.log("data is ",data);
        //alert(data)
    })

    socket.on("particular User",function(data){
        console.log("data from server ",data);
        //alert(data)
    })