Wikipedia
MassTransit is free software/ open-source .NET-based Enterprise Service Bus (ESB) software that helps Microsoft developers route messages over MSMQ, RabbitMQ, TIBCO and ActiveMQ service busses, with native support for MSMQ and RabbitMQ. It supports multicast, versioning, encryption, sagas, retries, transactions, distributed systems and other features. It uses a "Control Bus" design to coordinate and the Rete algorithm to route. Since it does not include "business monitoring" or a " business rules engine" (and requires programming to implement sagas for orchestration), MassTransit is typically considered to be in the category of "lightweight ESB" software.
The project is led by Microsoft MVP and national conference speaker Chris Patterson ("phatboyg"), who is also the author of the Magnum and TopShelf projects, and is co-authored by Dru Sellers and Travis Smith. By April 2014 downloads of the package through NuGet passed 1,000 per week.
MassTransit is similar to a commercial offering called nServiceBus, and developers often pick one or the other for their implementation. The similarity is no accident, as the authors note that MassTransit was first built in 2007 as an alternative to nServiceBus, and the projects share the use of some code. Other similar "NuGet" ESB packages include Rebus and Rhino Service Bus.