Open Source tools are important to understand and use in order build applications quickly. Here's a list that we commonly use and highly recommend.
Subversion and Git are source code management systems used to keep the revisions accessible and allow for developer collaboration.
NUnit and Rhino.Mocks are tools for unit testing code to ensure it is operating correctly continues to do so in the future.
Spark View Engine and ASP.NET MVC are used to build the user interface layer on the server.
NHibernate is an object relational mapper, which helps reduces the time it takes to write data centric applications. Fluent NHibernate further reduces that time by allowing quick iterations of the data design mapping NHibernate with conventions.
The Castle Project is a framework and set of components that allow for quicker integration of various software building blocks like, NHibernate, log4net, Spark View Engine, Windows Communication Foundation (WCF), etc.
Log4net is a logging framework used for diagnostics.
NServiceBus is used create powerful service orientated applications by providing a messaging framework used to decouple integrated systems.