Basics

Get the id of the current resource

$modx->resource->id

Get User Info

$user = $modx->getUser();
echo $user->get('username');
Available fields:
id,username,password,cachepwd,class_key,active,remote_key,remote_data,hash_class,
salt,primary_group, session_stale, sudo

To find more useful information

$profile = $modx->user->getOne('Profile');
$fullname = $profile->get('fullname');
Available fields: fullname, email, phone, blocked, blockeduntil, blockedafter, logincount, lastlogin, thislogin
failedlogincount, sesionid, dob, gender, address, country, city, state, zip, fax, photo, comment, website, extended

Basic database query:

$query = 'select name from myTable where id = '.$id;
$result = $modx->query($query);
if (!is_object($result)) {
return 'No result!';
}
else {
$row = $result->fetch(PDO::FETCH_ASSOC);
return 'Result:' .print_r($row,true);
}

$stmt = $modx->query('SELECT * FROM `mytable` ORDER BY `name` ASC');
if ($stmt && $stmt instanceof PDOStatement) {
    while ($row= $stmt->fetch(PDO::FETCH_ASSOC)) {
        echo $row['name'];
    }
}

...

Accessing a custom db that you have generated the schema and map files for (possibly using CMP Generator

      // Install package<br>      
$path = MODX_CORE_PATH . 'components/myModel/';
$result = $modx->addPackage('myModel',$path .'model/','');
$rows = $modx->getCollection('MyModel');      
$output .= '<p>Total: '. count($rows ) . '</p>';
foreach($rows as $row) {
  $count = 0; 
  $output .= 'id: ' . $row->get('id');;           
  $output .= '<br/>name: ' . $row->get('name');          
  $output .= '<br/></p>';           
  $children = $row->getMany('My Other Model Class That Has A Many To One Relationship With The Above Class');           
  $output .= '<p>Children: '. count($children) . '</p>';
  foreach($children as $child) {                
         // do stuff <br>           
  }       
}      
return $output;

...

Write something to the Error Log

$modx->log(modX::LOG_LEVEL_ERROR,'Something... ');

Gotchas!

Formit

When putting in your public and private keys in the system settings make sure you put it in formit.recaptcha_private_key and formit.recaptcha_public_key and NOT in the recaptcha.private_key and recaptcha.public_key. That can keep you confused for a while!