Some of the common choices are web services (lowest common denominator) or a message-oriented-middleware bus. We have both going on somewhat competitively. WebMethods is a neat integration platform for web services, MQ-Series and MQSI work nicely for messaging.
You also have to think about centralized or decentralized models. One group is trying to put WebMethods in front of many services - come to one server and one service organization for everything. Others pop up with their own
SOAP servers and 3rd party servers. They are not perfectly interoperable so the client has to know quite a bit about who they're talking to.
I talked with one well-known company that had an extremely large centralized hub with a goal of presenting the only public API for hundreds of services all over the planet, real time and batch. That kind of thing takes a huge commitment to reliability, avialability, scalability, performance, manpower and playing well with others. Don't go there without CIO mandates!