I'm looking for an object database for .NET that is open source and free for commercial use. I need something that is available as assemblies and/or source code to embed into a project, i.e. not a stand alone database in its own process.
I am aware of db4o, and it would be perfect except for the commercial license coming at a price.
ACID transaction support is desirable, but I really just want to know what options are out there.
What sort of objects do you need to store? One possibility is MongoDB, which lets you store documents containing JSON-like key-value pairs. There's a C#/.NET driver.
IMO the one you mention (db4O) is easily head and shoulders the best OODBMS out there. It simply just works how you expect, has Linq support and perf is very good. It is also free for you in development mode and free for use in open source projects.
And...for commercial use the licence is very attractive, typically you only pay on a (very small) percentage basis of your revenue. IMO for a commercial product it's totally worth it, your customers will also have the comfort of knowing your database solution is backed by someone like Versant and not some potentially forked or abandoned Open Source solution.
I know that you want free all the way, but honestly you will give up a lot for that, and the small price you'll pay for db4O (after you are making already making money from it) will be totally worth it.
(I do not have any affiliation with Versant or db4o, apart from being a very happy user)
Give a try to Eloquera Database (www.eloquera.com) It's FREE for commercial use and native .NET object database.
Try Scalable Object Persistence (SOP) framework from http://www.4atech.net!