UserFields for TYPO3 (TCA)

class.tx_ext_myCustomField.php

<?php

class tx_ext_myCustomField {
	/**
	 * @var array Field parameters
	 */
	protected $parameter = array();

	/**
	 * @param $parameter
	 * @return string
	 */
	function getMyCustomField($parameter) {
		$isCool = $parameter['fieldConf']['config']['parameters']['isCool'];

		return '<div>The HTML of your field</div>';
	}

}

ext_localconf.php

<?php
if (!defined('TYPO3_MODE')) {
    die ('Access denied.');
}

if (TYPO3_MODE === 'BE') {
	require_once(t3lib_extMgm::extPath($_EXTKEY).'Classes/UserFunction/class.tx_ext_myCustomField.php');
}

YourTCA.php

<?php
if (!defined ('TYPO3_MODE')) {
	die ('Access denied.');
}

$TCA['tx_ext_domain_model_whatever'] = array(
	'ctrl' => $TCA['tx_ext_domain_model_whatever']['ctrl'],
	'interface' => array(
		'showRecordFieldList' => 'sys_language_uid, l10n_parent, l10n_diffsource, hidden',
	),
	'types' => array(
		'1' => array('showitem' => 'hidden;;1,myCoolNewOwnField'),
	),
	'palettes' => array(
		'1' => array('showitem' => ''),
	),
	'columns' => array(
		'template_content' => array(
			'exclude' => 0,
			'label' => 'My cool new own field',
			'config' => array (
				'type' => 'user',
				'userFunc' => 'EXT:ext/Classes/UserFunction/class.tx_ext_myCustomField.php:tx_ext_myCustomField->getMyCustomField',
				'parameters' => array(
					'isCool' => TRUE
				),
			),
		),
	),
);