Some Interesting and important built in packages in Oracle

DBMS_JOB : To schedule jobs.

DBMS_SCHEDULER: Extended the functionalities of DBMS_JOB.

UTL_SMTP: To send mails.

UTL_HTTP: To open Web APIs (URLs).

DBMS_LOB: Very interesting and useful. Programmatic interface to deal with Large objects like Clob, Nclob, Blob and Bfile

DBMS_UTILITY: Wide range of functionalities. Check format_error_backtrace, format_call_stack, format_error_stack etc.

UTL_FILE: File handling.

DBMS_REDIFINITION: Online redefinition of tables. To partition an existing table etc

DBMS_WORKLOAD_REPOSITORY: Used in performance tuning to generate/drop snapshopts, to create/remove baselines, to change snapshot settings etc which can be further used in generating the AWR reports.

DBMS_SQL: Dynamic SQL support

DBMS_XPLAN: Format the output of explain plan.