Modules to enhance CCK

All these modules require CCK.

Additional field types

Date (http://drupal.org/project/date)
Recommends jQuery UI for popup calendars and time entry widgets.
Defines CCK date/time fields and widgets. What makes this module essential is that it also provides a Date API submodule that provides superior date & time handling than in Drupal core.(This functionality is part of core in Drupal 7.)
FileField (http://drupal.org/project/filefield)
Defines a file field type for CCK and enables uploading files.(This functionality is part of core in Drupal 7.)
ImageField (http://drupal.org/project/imagefield)
Requires FileField.
Defines an image field type for CCK and improves uploading images.(This functionality is part of core in Drupal 7.)
Link (http://drupal.org/project/link)
Defines link field types. These allow association of a link title with a link URL, while permitting either to be optional.
Email Field (http://drupal.org/project/email)
Defines an email field type for CCK
Phone (CCK) (http://drupal.org/project/phone)
Allows administrators to define a CCK field type for phone numbers.
Viewfield (http://drupal.org/project/viewfield)
Defines a field type that displays the contents of a view in a node.

Additional field widgets

Multiselect (http://drupal.org/project/multiselect)
Defines a CCK multiple selection field widget, to allow easier multi-selection for users. [Accessibility still needs to be evaluated (e.g, by John Foliot of SOAP).]
Autocomplete Widgets (http://drupal.org/project/autocomplete_widgets)
Provides autocomplete widgets for CCK text and number fields.