What to use for Messaging with C#

ActiveMQ works well with C# using the Spring.NET integrations and NMS. A post with some links to get you started in that direction is here. Also consider using MSMQ (The System.Messaging namespace) or a .NET based asynchronous messaging solution, with some options here.

MSMQ (Microsoft Message Queueing) may be a great choice. It is part of the OS and present as an optional component (can be installed via Add/Remove Programs / Windows Components), meaning it's free (as long you already paid for Windows, of course). MSMQ provides Win32/COM and System.Messaging APIs. More modern Windows Communication Foundation (aka Indigo) queued channels also use MSMQ. Note that MSMQ is not supported on Home SKUs of Windows (XP Home and Vista Home)