A Wordpress widget base class which reduces the amount of code needed to create a simple widget with fields. Allows overriding a method adding more complex fields. This code should be placed in your theme's function.php file. You can then subclass Field_Widget whenever you like by following the example given.

field_widget.php

/**
 * A Wordpress widget base class which reduces the amount of code needed to create a simple widget with fields. Allows overriding a method adding more complex fields.
 */
class Field_Widget extends WP_Widget {

    private $fields = array();
    private $defaults = array();
    private $names = array();

    function update($new_instance, $old_instance) {
        $instance = $old_instance;
        foreach ($new_instance as $field => $value) {
            $instance[$field] = strip_tags($value);
        }
        return $instance;
    }

    /**
     * Delegates the content of the widget to widget_content() and prints the title and surrounding content
     * Override this if needed for more customisation.
     *
     * @param $args
     * @param $instance
     */
    function widget($args, $instance) {
        echo $args['before_widget'];
        echo $args['before_title'];
        $this->widget_title($args, $instance);
        echo $args['after_title'];
        $this->widget_content($args, $instance);
        echo $args['after_widget'];
    }

    function widget_title($args, $instance) {
        echo $args['widget_name'];
    }

    /**
     * Used to print the widget content
     *
     * @param $args
     * @param $instance
     */
    function widget_content($args, $instance) {
        // Override!
    }

    /**
     * Sets form fields.
     *
     * @param $fields An associative array of field ids to either values or an array with
     * the first element as the field name and the second as the field value
     */
    function set_fields($fields) {
        $this->fields = $fields;
        foreach ($fields as $field => $value) {
            if (is_array($value)) {
                $this->names[$field] = $value[0];
                $this->defaults[$field] = $value[1];
            } else {
                $this->names[$field] = ucwords($value);
                $this->defaults[$field] = $value;
            }
        }
    }

    function get_defaults() {
        return $this->defaults;
    }

    function get_fields() {
        return $this->fields;
    }

    function get_names() {
        return $this->names;
    }

    /**
     * Called for each form field encountered in form().
     * Override and call parent::form_field() in subclasses when default behaviour is needed.
     *
     * @param $field The id of the form field defined with set_fields()
     * @param $name The name of the form field defined with set_fields()
     * @param $value The value of field from the db; blank on first submit
     * @param $id The generated form field id
     */
    function form_field($field, $name, $value, $id) {
        ?>
    <p><label for="<?php echo $id ?>"><?php echo $name; ?>:</label>
        <input class="widefat" id="<?php echo $id; ?>"
               name="<?php echo $this->get_field_name($field); ?>"
               type="text"
               value="<?php echo attribute_escape($value); ?>"/>
    </p>
    <?php
    }

    /**
     * Overridden form function which delegates drawing the form to form_field()
     */
    function form($instance) {
        $defaults = self::get_defaults();
        $names = self::get_names();
        $instanceArray = wp_parse_args((array)$instance, $defaults);
        foreach (array_keys($defaults) as $field) {
            $name = isset($names[$field]) ? $names[$field] : ucwords($field);
            $value = strip_tags($instanceArray[$field]);
            $id = $this->get_field_id($field);
            self::form_field($field, $name, $value, $id);
        }
    }

}

/**
 * Widget which allows choosing a page or post and writing a few short descriptions about it.
 * The widget then presents the thumbnail image and the descriptions.
 */
class FeaturedPost extends Field_Widget {

    function __construct() {
        parent::__construct(false, 'Featured Post', array(
            'description' => 'Displays the featured image and descriptions for a post'
        ));
        self::set_fields(array(
            'post' => array(__('Post'), ''),
            'description' => array(__('Description'), ''),
            'small-description' => array(__('Small Description'), '')
        ));
    }

    function widget($args, $instance) {
        $id = intval($instance['post']);
        $desc = $instance['description'];
        $smallDesc = $instance['small-description'];
        $post = get_post($id);
        if ($post) {
            echo '<div class="featured-post">';
            echo '<div class="header"><h3>' . get_the_title($id) . '</h3></div>';
            $thumbnail = get_the_post_thumbnail($id, array(275, 207));
            echo '<div class="thumbnail">' . $thumbnail . '</div>';
            if ($desc) {
                echo '<div class="description">' . $desc . '</div>';
            }
            if ($smallDesc) {
                echo '<div class="small-description">' . $smallDesc . '</div>';
            }
            echo '</div>';
        }
    }

    function form_field($field, $name, $value, $id) {
        if ($field == 'post') {
            ?>
        <p>
            <label for="<?php echo $id ?>"><?php echo $name; ?>:</label>
            <select class="widefat" id="<?php echo $id; ?>" name="<?php echo $this->get_field_name($field); ?>">
                <?php
                $posts = get_all_posts();
                foreach ($posts as $post) {
                    echo '<option value="' . $post->ID . '" ' . selected($value, $post->ID, false) . '>' . get_the_title($post->ID) . '</option>';
                }
                ?>
            </select>
        </p>
        <?php
        } else {
            parent::form_field($field, $name, $value, $id);
        }
    }
}

register_widget('FeaturedPost');

/**
 * Widget which shows the last several posts with thumbnails
 */
class ThumbnailPosts extends Field_Widget {

    function __construct() {
        parent::__construct(false, 'Thumbnail Posts', array(
            'description' => 'Displays the last several posts with thumbnails'
        ));
        self::set_fields(array(
            'title' => array(__('Title'), ''),
            'limit' => array(__('Limit'), 2)
        ));
    }

    function widget_content($args, $instance) {
        $limit = intval($instance['post']);
        $posts = get_posts(array(
            'post_type' => 'post',
            'orderby' => 'post_date',
            'limit' => $limit
        ));
        foreach ($posts as $post) {
            $id = $post->ID;
            echo '<div class="thumbnail-post">';
            echo '<div class="header"><h4>' . get_the_title($id) . '</h4></div>';
            $thumbnail = get_the_post_thumbnail($id, array(275, 207));
            echo '<div class="thumbnail">' . $thumbnail . '</div>';
            echo '</div>';
        }
    }

    function widget_title($args, $instance) {
        echo $instance['title'];
    }

    function form_field($field, $name, $value, $id) {
        if ($field == 'post') {
            ?>
        <p>
            <label for="<?php echo $id ?>"><?php echo $name; ?>:</label>
            <select class="widefat" id="<?php echo $id; ?>" name="<?php echo $this->get_field_name($field); ?>">
                <?php
                $posts = array_merge(get_posts(array(
                    'orderby' => 'post_date',
                    'post_type' => 'any'
                )), get_posts(array(
                    'orderby' => 'post_date',
                    'post_type' => 'post'
                )));
                foreach ($posts as $post) {
                    echo '<option value="' . $post->ID . '" ' . selected($value, $post->ID, false) . '>' . get_the_title($post->ID) . '</option>';
                }
                ?>
            </select>
        </p>
        <?php
        } else {
            parent::form_field($field, $name, $value, $id);
        }
    }
}

register_widget('ThumbnailPosts');