Maverick is an open-source version of a Pick-like MultiValue database written in Java. I've seen old Pick systems but never could work out whether it was an alternative to an OS or an alternative to SQL databases.
""You can think in a relational manner, you can program it like a relational database, but it doesn't [have] all the relational constraints," Youngman said. For example, programming in SQL requires tables for each line on an invoice, while Pick needs just a single record per invoice that can be viewed relationally, he said." Open-source Pick-like database being developed
It currently backends onto some existing relational databases like MySQL.