Where can I a list of available (in the core) fields and widgets and their names so I can reference them when adding fields to my own node-type definitions ?
To get a list of all field definitions use
Returns all field definitions.
Use of this function should be avoided when possible, since it loads and statically caches a potentially large array of information. Use field_info_field_map() instead.
You can ignore the second paragraph of that as
field_info_field_map() hasn't actually been included in a Drupal release yet.
For widgets use
Returns information about field widgets from hook_field_widget_info().