Often misunderstood by people new to SQL, the WITH clause (known in other databases as Common Table Expressions or CTE's, but in Oracle is known as the Subquery Factoring Clause), aside from being difficult to find in the documentation (if you don't know where to look), has many good uses. The attached PDF document details the uses of WITH with some examples.
PLSQL 101 - WITH Clause.pdf
(Edit: Now re-uploaded for new forum platform)