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

GemBox.Spreadsheet - GemBox.Spreadsheet makes working with Excel files enjoyable. You can use a free version if you have 150 or fewer rows in your spreadsheet.

IOC (Inversion of Control) Container

Ninject

Structure Map

Logging

NLog

log4net

Scheduling

Quartz .NET

Testing

Approval Tests

AutoFixture

FakeItEasy - A dead simple to use "mocking/stubbing" library where you just make "fakes".

Fixie

Validation

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.