Materialized views are great and (obviously) useful, but they have the usual tradeoffs of any caching mechanism (e.g. now you have to worry about cache data age and invalidation.

IMO a slept-on database feature is table partitioning to improve query performance. If you have a frequently-used filter field that you can partition on (e.g. creation timestamp), then you can radically improve query performance of large databases by having the DB only need to full-scan the given partitions. The database itself manages where records are placed, so there is no additional overhead complexity beyond initial setup. I've only used this for PostgreSQL, but I assume that other databases have similar partition mechanisms.