This is one of the .NET interview questions asked in most of the interviews.
1) Factory Pattern: Used for de-coupling entities. For eg. decouple UI from middle tier or like decouple middle tier from data access.
2) Abstract factory pattern groups Factory Pattern logically.
3) Singleton: Used for caching and global sharing of data.
4) WCF: Used to communicate with non-microsoft like Java or CGI.
5) Restful service: When you want to communicate with cross platform clients and cross platform clients do not have ability to consume complicated SOAP messages.
Also view the following Design Pattern video on Lazy Loading