This Google Apps Script will turn Google Docs into a website uptime monitor.

/**   Monitor your Site's Uptime  **/

function isMySiteDown() {
  // Get the URL of the Website to monitor
  var url = SpreadsheetApp.getActiveSheet().getRange('E3').getValue();

  // HTTP Response Code of the last server request
  if (!ScriptProperties.getProperty('status')) {
    ScriptProperties.setProperty('status', 200);
  }

  var response, error;

  try {
    // Fetch the web page using UrlFetchApp
    response = UrlFetchApp.fetch(url);
  } catch (error) {
    insertData(error, -1, 'Website down');
    return;
  }

  var code = response.getResponseCode();

  // code = 200 means the fetch request was successful
  if (code == 200) insertData('Up', code, 'Website up');
  else insertData(response.getContent()[0], code, 'Website down');
}

function insertData(error, code, msg) {
  // Ignore if the error message is logged already
  if (ScriptProperties.getProperty('status') == code) return;

  // Log the server error in a Google Sheet

  var sheet = SpreadsheetApp.getActiveSheet();
  var email = sheet.getRange('E5').getValue();
  var row = sheet.getLastRow() + 1;

  sheet.getRange(row, 1).setValue(new Date());
  sheet.getRange(row, 2).setValue(error);
  sheet.getRange(row, 3).setValue(code);

  // Send an email alert for the downtime

  ScriptProperties.setProperty('status', code);
  MailApp.sendEmail(email, msg, error);
}