Without more details of what you want to do – as Henry has said – it is hard to point you at anything in particular.
But let’s assume you are working for a publisher and wish to create an ONIX file detailing your products. It’s worth saying at the outset that approaching ONIX as primarily a technical problem is usually a mistake. It is important to understand that ONIX is not something you implement just by downloading some software. Understanding the business and organisational issues is usually more difficult.
ONIX is a specification for communication of rich metadata between organisations. So the first task is to ensure that the publishing organisation is collecting and managing the relevant metadata – ideally in some database, which can of course be based on an open-source DBMS. That database is likely to be vital to internal communication and management, to feed your own website, to support your planning, production, marketing or distribution etc. Once this data is being collected and managed in a consistent and timely way, using the database to extract the data and format it into an ONIX message is -- at least in principle – relatively straightforward. ONIX is based on embedding data from that database XML, and there’s no end to the open-source XML tools that can be used to help, and most DBMSs have an API offering XML output.
But there’s nothing you can just download, install and get ONIX in one go. Each company is different, and the selection of data that you will put in your ONIX depends somewhat on the nature of your business, the requirements of your business partners and on how you built the underlying database. You’ll need to do the analysis to decide which parts of ONIX are most important to your business and its supply-chain partners. You’ll need to create the underlying database, and ensure your business process keeps that data accurate and up-to-date. And you’ll need to write some code to extract data from your database and format it into ONIX. Each step might well be built on some open-source material, but there’s no one-size-fits-all solution.
You should also look at various of the commercial offers too, though – there are many locally-installable applications and cloud-based services that will help with management of your publishing programme and various other parts of your business process, and they then also produce ONIX to communicate with others. Some of these commercial offers have a relatively low cost of entry, particularly for small publishers. You can see some of them listed on EDItEUR’s users and services directory.