Salesforce Apex Design Patterns – Biswajeet Samal's Blog

Singleton: Minimizing object instantiation for improved performance and to mitigate impact of governor limits.
Strategy: Defining a family of algorithms, enscapsulating each one and making them interchangeable and selectable at runtime.
Decorator: Extending the functionality of an sObject in Apex.
Facade: Simplifying the execution of classes with complex interfaces (e.g. web service callouts).
Composite: Treating a group of objects in a similar manner to a single instance of that object.
Bulk State Transition: Efficiently tracking the change of a field value in a trigger and executing functionality based on this change.