An open reuse of public data and tools can turn the government into a powerful ‘platform’ also involving external innovators. However, the typical information system of a public agency is not open by design. Several public administrations have started adopting technical solutions to overcome this issue, typically in the form of middleware layers operating as ‘buses’ between data centres and the outside world. Open-DAI is an open source platform designed to expose data as services, directly pulling from legacy databases of the data holder. The platform is the result of an ongoing project funded under the EU ICT PSP call 2011. We present the rationale and features of Open-DAI, also through a comparison with three other open data platforms: the Socrata Open Data portal, CKAN, and ENGAGE.