We don't call it an ESB in our non-Enterprise environment. It's just called "service bus" or "message bus". Which is really just a fancy application-level API around services, queues, and messages (see http://particular.net/NServiceBus). It's incredibly useful, dare I say essential, for abstracting away those infrastructure level concerns when developing complex distributed applications.

We use it pretty extensively in our SaaS product to achieve loose coupling, yet high cohesion, between our various software components.