Which are the core optional modules in drupal 6.x ?

Answer

 1. Aggregator Aggregates syndicated content (RSS, RDF, and Atom feeds).

2. Blog Enables keeping easily and regularly updated user web pages or blogs.

3. Blog API Allows users to post content using applications that support XML-RPC blog APIs.

4. Book Allows users to structure site pages in a hierarchy or outline.

5. Color Allows the user to change the color scheme of certain themes.

6. Comment Allows users to comment on and discuss published content.

7. Contact Enables the use of both personal and site-wide contact forms.

8. Content translation Allows content to be translated into different languages.

9. Database logging Logs and records system events to the database.

10. Forum Enables threaded discussions about general topics.

11. Help Manages the display of online help.
12. Locale Adds language handling functionality and enables the translation of the user interface to languages other than English.

13. Menu Allows administrators to customize the site navigation menu.
14. OpenID Allows users to log into your site using OpenID.
15. Path Allows users to rename URLs.
16. PHP filter Allows embedded PHP code/snippets to be evaluated.
17. Ping Alerts other sites when your site has been updated.
18. Poll Allows your site to capture votes on different topics in the form of multiple choice questions.
19. Profile Supports configurable user profiles.
20. Search Enables site-wide keyword searching.
21. Statistics Logs access statistics for your site.
22. Syslog Logs and records system events to syslog.
23. Taxonomy Enables the categorization of content.
24. Throttle Handles the auto-throttling mechanism, to control site congestion.
25. Tracker Enables tracking of recent posts for users.

26. Trigger Enables actions to be fired on certain system events, such as when new content is created.
27. Update status Checks the status of available updates for Drupal and your installed modules and themes.
28. Upload Allows users to upload and attach files to content.