What are the default core modules

The following modules are default modules available in Drupal 6.x version.
modules colored by green is default selected modules, they are automatically get installed when Drupal installation.

* Aggregator => Aggregates syndicated content (RSS, RDF, and Atom feeds).
* Blog => Enables keeping easily and regularly updated user web pages or blogs.
* Blog API => Allows users to post content using applications that support XML-RPC blog APIs.
* Book => Allows users to structure site pages in a hierarchy or outline.
* Color => Allows the user to change the color scheme of certain themes.
* Comment => Allows users to comment on and discuss published content.
* Contact => Enables the use of both personal and site-wide contact forms.
* Content translation => Allows content to be translated into different languages.
* Database logging => Logs and records system events to the database.
* Forum => Enables threaded discussions about general topics.
* Help => Manages the display of online help.
* Locale => Adds language handling functionality and enables the translation of the user interface to languages other than English.
* Menu => Allows administrators to customize the site navigation menu.
* OpenID => Allows users to log into your site using OpenID.
* Path => Allows users to rename URLs.
* PHP filter => Allows embedded PHP code/snippets to be evaluated.
* Ping => Alerts other sites when your site has been updated.
* Poll => Allows your site to capture votes on different topics in the form of multiple choice questions.
* Profile => Supports configurable user profiles.
* Search => Enables site-wide keyword searching.
* Statistics => Logs access statistics for your site.
* Syslog => Logs and records system events to syslog.
* Taxonomy => Enables the categorization of content.
* Throttle => Handles the auto-throttling mechanism, to control site congestion.
* Tracker => Enables tracking of recent posts for users.
* Trigger => Enables actions to be fired on certain system events, such as when new content is created.
* Update status => Checks the status of available updates for Drupal and your installed modules and themes.
* Upload => Allows users to upload and attach files to content.