GoF [Gang Of Four] design patterns used:

For forcing only one instance of Doctrine_Manager

For leveled configuration

For connection driver loading and many other things

For event listening

For efficient usage of validators

For iterating through components [Tables, Connections, Records etc.]

For state-wise connections

For algorithm strategies

Enterprise application design patterns used:

Doctrine is an implementation of this pattern

For maintaining a list of objects affected in a transaction

For maintaining the identity between record and database row

For Doctrine DataDict

For mapping in general, since all records extend Doctrine_Record which performs all mappings

For one-to-one, one-to-many and many-to-one relationships

For association table mapping (most commonly many-to-many relationships)

For lazy loading of objects and object properties

DQL API is actually an extension to the basic idea of Query Object pattern