How to use SignalR hub instance outside of the hubpipleline

Please note that the SignalR API has changed multiple times since this question was asked. There is a chance that some answers will become out of date. This does not mean that they should be down-voted as they were correct at the time of writing

There is another updated answer for this, as seen in the SignalR Wiki

c#

Public ActionResult MyControllerMethod()
{
    var context = GlobalHost.ConnectionManager.GetHubContext<MyHub>();
    context.Clients.All.methodInJavascript("hello world");
    // or
    context.Clients.Group("groupname").methodInJavascript("hello world");
}

vb.net

Public Function MyControllerMethod() As ActionResult
    Dim context = GlobalHost.ConnectionManager.GetHubContext(Of MyHub)()
    context.Clients.All.methodInJavascript("hello world")
    '' or
    context.Clients.Group("groupname").methodInJavascript("hello world")
End Function

Update

This code has been updated. Follow http://www.asp.net/signalr/overview/signalr-20/hubs-api/hubs-api-guide-server for changes.

If you are using DI container

If you are using a DI container to wire up your hubs, get IConnectionManager from your container, and call GetHubContext on that connectionManager.

This is the correct way for SignalR 2.x:

var context = GlobalHost.ConnectionManager.GetHubContext<MyHub>();
context.Clients.All.addMessage(message);

Basically, you can use the dependency resolver for the current host to resolve the IConnectionManager interface which allows you to get ahold of the context object for a hub.

Further information can be found in the official documentation.