Gemstone has a free version limited to 4 gig, but few if any object databases are language agnostic, that would defeat the purpose. Object systems are language specific, storing them in their native format is thus generally also language specific.

There's an open source one called Magma in Squeak that's like a Gemstone lite, but I'd take the Gemstone version any day because it'll scale to any level you'll ever need, ever. I'm working on a Gemstone project now, and after having used it, nothing else I've seen comes close to the massive productivity it offers.

By the way, databases shouldn't be language agnostic, this leads to the ever present anti pattern of using the database as an integration point between many programs which turns the database into a giant ball of mud global variable that becomes impossible to change.

Integration databases suck. Application should own their own data and integrate with other application via services. That's how the web became so successful and that's how big ass enterprises should be ran as well, many small apps loosely coupled, not one giant global db where every app is bound to a generic schema that isn't suited for what it actually needs.