Service-oriented architecture is an idea, not a technology. Boundless in scope, it promises both unlimited software reuse and the interconnection of everything, as long as IT is willing to wrap legacy applications in standard interfaces and construct new apps as services, the capabilities of which other software can tap into.

The idea is simple, but the execution isn?t, because SOA turns the conventional model of enterprise software development on its head. Normally, programmers write software based on a set of well-defined requirements. SOA demands that organizations create an ecosystem of services that may ultimately have an army of stakeholders inside and outside the firewall. The initial challenge of SOA is knowing where and how to start -- where to draw a box around a fixed set of requirements and how to build services that will yield tangible ROI while keeping an SOA fully extensible.

For this article, we evaluated dozens of SOA implementations to find five that had a major impact on an enterprise and/or its partners: British Telecom, Countrywide Financial, Guardian Life Insurance, the New England Healthcare EDI Network, and Transamerica Life Insurance. These projects are largely works in progress; some are only in their initial phases of implementation. But all can help light the way for enterprises in search of their own strategies to make a simple, powerful idea come to life.