In order to build software quickly we stand on the shoulder of giants. The .NET ecosystem has many great software packages that can be leveraged to speed up your development cycle. Here is a list of packages available via NuGet that will land you in the pit of success if you use them.
Excel File Manipulation
IOC (Inversion of Control) Container
FakeItEasy - A dead simple to use "mocking/stubbing" library where you just make "fakes".
Fluent Validation - A great library for producing readable and testable validation code.
Misc. .NET Libraries
Automapper - Automapper is great at reducing the need for repetitive mapping code when moving data from your domain entities to your view models.
Enumeration - The built in enums in .NET are often not flexible enough for business applications. The Enumeration class is a more flexible enum implementation that can handle the extra metadata you often need with a fixed value.