This Google Apps Script will sift through your Gmail account and extract email addresses of senders which are then saved in a Google Sheet. Useful for email marketing and mail merge.

//   Written by Amit Agarwal on 06/13/2013
function extractEmailAddresses() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheets()[0];

  var monitor = sheet.getRange('A2').getValue();
  var processed = sheet.getRange('B2').getValue();

  var label = GmailApp.getUserLabelByName(processed);
  var search = 'in:' + monitor + ' -in:' + processed;

  // Process 50 Gmail threads in a batch to prevent script execution errors
  var threads = GmailApp.search(search, 0, 50);

  var row, messages, from, email;

  try {
    for (var x = 0; x < threads.length; x++) {
      // Use Regular Expression to extract valid email address
      from = threads[x].getMessages()[0].getFrom();
      from = from.match(/\S+@\S+\.\S+/g);

      if (from.length) {
        email = from[0];
        email = email.replace('>', '');
        email = email.replace('<', '');

        row = sheet.getLastRow() + 1;
        // If an email address if found, add it to the sheet
        sheet.getRange(row, 1).setValue(email);
      }

      threads[x].addLabel(label);
    }
  } catch (e) {
    Logger.log(e.toString());
    Utilities.sleep(5000);
  }

  // All messages in the label have been processed?
  if (threads.length === 0) {
    GmailApp.sendEmail(Session.getActiveUser().getEmail(), 'Extraction Done', 'Download the sheet from ' + ss.getUrl());
  }
}

// Remove Duplicate Email addresses
function cleanList() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var data = sheet.getRange(4, 1, sheet.getLastRow()).getValues();
  var newData = new Array();
  for (i in data) {
    var row = data[i];
    var duplicate = false;
    for (j in newData) {
      if (row[0] == newData[j][0]) {
        duplicate = true;
      }
    }
    if (!duplicate) {
      newData.push(row);
    }
  }

  // Put the unique email addresses in the Google sheet
  sheet.getRange(4, 2, newData.length, newData[0].length).setValues(newData);
}