Lately I've been learning how to use an ESB that my company (Neudesic) created. I'm really liking it, it seems very intuitive and easy to use. It has a bit of overlap with BizTalk functionality, but I think that both of them together make a great team. I'm not very fond of the BizTalk ESB Toolkit, so I'm glad to find something I like better.
The product is called Neuron ESB. I don't have time to go into detail at the moment (deadlines, you know!), but I plan to write more later. You can click on the link for more details.