From the Database Geek.

Part of the ANSI SQL standard calls for an INFORMATION_SCHEMA. This schema contains a standardized data dictionary that is (or is supposed to be) common across various databases. Most database vendors offer a native data dictionary and a sub-set INFORMATION_SCHEMA (called info schema from this point on). Oracle is the only major database vendor (that I know of) that doesn’t even offer a subset of the info schema.

Oracle’s data dictionary (especially if you include the V$ views in that) is the most robust of all the databases I have used.