A funny little hack I put together over the weekend. Read this for more information: http://ping.marcchung.com

Ping.js

/**
 * Each record captured by the form is represented as a PingAlert.
 */
function PingAlert(rowId, record) {
  this.rowNum = rowId + 1;
  // See Spreadsheet for information on index.
  this.url = record[1];
  this.email = record[2];
  
  // Extra columns
  this.active = record[3]; // Live
  this.accumDowntime = parseInt(record[4], 10); // Accumulated downtime
  this.nextOutgoingAlert = parseInt(record[5], 10); // Next downtime alert dispatched
  this.alertsDispatched = parseInt(record[6], 10); // Running count of alerts dispatched
}

/**
 * Does the actual UrlFetch/Ping. Will depend on cached copy for subsequent calls.
 */
PingAlert.prototype.isAvailable = function() {
  if (this.response === undefined) {
    var options = {
      "muteHttpExceptions" : true
    };
    this.response = UrlFetchApp.fetch(this.url, options);
  }
  return this.response.getResponseCode() == 200;
}

PingAlert.prototype.firstAlert = function() {
  return (this.accumDowntime === 0 && this.nextOutgoingAlert === 0 && this.alertsDispatched === 0);
}

/**
 * Main engine. Update invariants, send out emails.
 */
PingAlert.prototype.dispatch = function() {
  if (this.isAvailable()) {
    // reset to initial state
    if (!this.firstAlert()) {
      this.sendUptimeAlert();
      this.accumDowntime = 0;
      this.nextOutgoingAlert = 0;
      this.alertsDispatched = 0;
    }
  } else {
    if (this.firstAlert()) {
      this.sendDowntimeAlert();
      this.accumDowntime = 0;
      this.nextOutgoingAlert = 1;
    } else {
      this.accumDowntime++;
      if (this.accumDowntime === this.nextOutgoingAlert) {
        this.sendDowntimeAlert();
        this.updateNextOutgoingAlert();
      }
    }
  }
}

/**
 * Save state till next time main() is triggered. Is there some 2-way binding API?
 */
PingAlert.prototype.save = function(sheet) {
  var updated = [[this.accumDowntime, this.nextOutgoingAlert, this.alertsDispatched]];
  sheet.getRange(this.rowNum, 5, 1, 3).setValues(updated);
}

/**
 * Fibonacci-like back off sequence to rate limit outgoing downtime emails.
 */
PingAlert.prototype.updateNextOutgoingAlert = function() {
  var phi = Math.pow(5, 0.5) * 0.5 + 0.5;
  var nextfib = Math.pow(phi, this.alertsDispatched+1) / Math.pow(5, 0.5);
  this.nextOutgoingAlert = Math.round(nextfib);
}

/**
 * Generates and delivers downtime email.
 */
PingAlert.prototype.sendDowntimeAlert = function() {
  Logger.log(this.url + " is down. Alerting " + this.email);
  var subject = "[Ping-a-lert] Uh-oh, " + this.url + " is unavailable";
  
  var output = ContentService.createTextOutput();
  output.append("Ahoy, " + this.email + "!");
  output.append("\n\n");
  output.append(this.url);
  if (this.firstAlert()) {
    output.append(" is down :(");
  } else {
    output.append(" is still down. ");
    output.append("\n\n");
    output.append("Total downtime: ");
    output.append(this.accumDowntime)
    if (this.accumDowntime == 1) {
      output.append(" minute.");
    } else {
      output.append(" minutes.");
    }
  }

  output.append("\n\n");

  output.append("Response headers:\n");
  var headers = this.response.getAllHeaders();
  for (var key in headers) {
    if(headers.hasOwnProperty(key)) {
      output.append(key + ": " + headers[key]);
      output.append("\n");
    }
  }
  output.append("\n\n");
  output.append("Response body:\n");
  output.append(this.response.getContentText());
  
  output.append("\n\n");
  output.append("Thanks,\n\n--\nPing-a-lert by Marc Chung -- http://github.com/mchung");
  MailApp.sendEmail(this.email, subject, output.getContent());
  this.alertsDispatched++;
}


/**
 * ... and we're back
 */
PingAlert.prototype.sendUptimeAlert = function() {
  Logger.log(this.url + " is back up. Alerting " + this.email);
  var subject = "[Ping-a-lert] Hooray, " + this.url + " is available";
  
  var output = ContentService.createTextOutput();
  output.append("Ahoy, " + this.email + "!");
  output.append("\n\n");
  output.append(this.url + " is back up again :)");

  output.append("\n\n");
  output.append("Total downtime: ");
  output.append(this.accumDowntime)
  if (this.accumDowntime == 1) {
    output.append(" minute.");
  } else {
    output.append(" minutes.");
  }
  output.append("\n\n");

  output.append("Response headers:\n");
  var headers = this.response.getAllHeaders();
  for (var key in headers) {
    if(headers.hasOwnProperty(key)) {
      output.append(key + ": " + headers[key]);
      output.append("\n");
    }
  }

  output.append("\n\n");
  output.append("Thanks,\n\n--\nPing-a-lert by Marc Chung -- http://github.com/mchung");
  MailApp.sendEmail(this.email, subject, output.getContent());
  this.alertsDispatched++;
}


/**
 * Ping-a-lert.
 */
function main() {
  Logger.log("Emails remaining: " + MailApp.getRemainingDailyQuota());

  var sheet = SpreadsheetApp.getActiveSheet();
  var rows = sheet.getDataRange();
  var numRows = rows.getNumRows();
  var values = rows.getValues();
  var sites = new Array();

  for (var i = 1; i <= numRows-1; i++) {
    var site = new PingAlert(i, values[i]);
    if (site.active) {
      site.dispatch();
      site.save(sheet);
    }
  }
}

/**
 * Adds a custom menu to the active spreadsheet, containing a single menu item
 * for invoking the readRows() function specified above.
 * The onOpen() function, when defined, is automatically invoked whenever the
 * spreadsheet is opened.
 * For more information on using the Spreadsheet API, see
 * https://developers.google.com/apps-script/service_spreadsheet
 */
function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = [{
    name : "Ping all active sites",
    functionName : "main"
  }];
  sheet.addMenu("Script Center Menu", entries);
};