In 10g there is a new view called DBA_FEATURE_USAGE_STATISTICS which keeps features have been used.

Here is description of some of the columns:
name – feature name
first_usage_date – first date when the feature was used
last_usage_date – latest date when it was detected the feature was used
last_sample_date – latest date when the sample was taken
last_sample_period – time in seconds between sample taken at last_sample_date and sample before last_sample_date
currently_used – value is TRUE or FALSE, TRUE if the feature was used since it was last checked

SQL> SELECT name, first_usage_date, last_usage_date, last_sample_date, last_sample_period FROM dba_feature_usage_statistics WHERE currently_used = ‘TRUE’;

NAME FIRST_USA LAST_USAG LAST_SAMP
—————————————- ——— ——— ———
Locally Managed Tablespaces (user) 30-SEP-08 08-FEB-09 08-FEB-09
Multiple Block Sizes 25-NOV-08 08-FEB-09 08-FEB-09
Partitioning (system) 30-SEP-08 08-FEB-09 08-FEB-09
Protection Mode – Maximum Performance 30-SEP-08 08-FEB-09 08-FEB-09
Segment Advisor 07-OCT-08 08-FEB-09 08-FEB-09
Streams (system) 30-SEP-08 08-FEB-09 08-FEB-09
Virtual Private Database (VPD) 30-SEP-08 08-FEB-09 08-FEB-09
Change-Aware Incremental Backup 02-DEC-08 08-FEB-09 08-FEB-09
Recovery Area 30-SEP-08 08-FEB-09 08-FEB-09
Streams (user) 30-SEP-08 08-FEB-09 08-FEB-09