This study describes the implementation of open source software in a large Irish public sector organization, Beaumont Hospital. The findings reveal a radical shift in open source deployment from invisible horizontal infrastructure systems to highly visible vertical applications. The case study describes the implementation of these systems, the difficulties encountered, and also the benefits in terms of astonishing cost savings of #13m over 5 years. These details are useful in that few studies have thus far quantified the savings from the deployment of open source software (OSS). Given that Beaumont were already receiving academic pricing discounts for many of their original proprietary closed source applications, the savings for a typical commercial organization could be even higher. Also, in many cases, the extra functionality available in the OSS systems deployed allowed for a richer feature set overall. The study also identifies the primary drivers in the move to OSS, namely principle, pragmatism and practicality. This is useful, since while several studies have considered the motivation of OSS developers, the motivation of actual users of OSS has received far less attention. The study also indicates how a typical company can contribute back to the OSS community in their own unique way, by distributing applications form their own domain of expertise, rather than having to make detailed code contributions to the original code base.