Verify a recaptcha using CoffeeScript and node.js. This is not bulletproof, but it is a start.

recaptcha.coffee

# Send a request to recaptcha to verify the user's input.
#
# The first argument must be an object containing four parts:
#    privatekey: Your recaptcha private key.
#    remoteip:   The IP of the user who submitted the form.
#    challenge:  The challenge value from the recaptcha form.
#    response:   The user's response to the captcha.
#
# Example usage (express):
#
#   recaptcha = require 'recaptcha'
#
#   app.post '/comments', (req, res) ->
#       data =
#           privatekey: YOUR_PRIVATE_KEY
#           remoteip:   req.connection.remoteAddress
#           challenge:  req.body.recaptcha_challenge_field
#           response:   req.body.recaptcha_response_field
#
#       recaptcha.verify_captcha data, (success, error_code) ->
#           if success
#               # Passed captcha.
#           else
#               # Did not pass captcha.

http        = require 'http'
querystring = require 'querystring'

API_HOST      = 'www.google.com'
API_END_POINT = '/recaptcha/api/verify'

exports.verify_captcha = (data, callback) ->
    data = querystring.stringify data

    recaptcha = http.createClient 80, API_HOST
    request = recaptcha.request 'POST', API_END_POINT,
        host:             API_HOST
        'Content-Length': data.length
        'Content-Type':   'application/x-www-form-urlencoded'

    request.on 'response', (response) ->
        body = ''
        response.on 'data', (chunk) ->
            body += chunk
        response.on 'end', () ->
            [success, error_code] = body.split '\n'
            callback(success == 'true', error_code)

    request.write data.toString(), 'utf8'
    request.end()