aggregator_form_opml in modules/aggregator/
Form constructor for importing feeds from OPML.
block_admin_configure in modules/block/
Form constructor for the block configuration form.
filter_admin_format_form in modules/filter/
Form constructor for the text format add/edit form.
node_form_search_form_alter in modules/node/node.module
Implements hook_form_FORM_ID_alter().
node_menu in modules/node/node.module
Implements hook_menu().
user_account_form in modules/user/user.module
Helper function to add default user account fields to user registration and edit form.
user_admin_account in modules/user/
Form builder; User administration page.
user_menu in modules/user/user.module
Implements hook_menu().