Really simple cross-platform database?

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http://www.unix.com – I'm looking for a really simple to use lightweight database. Ideally something open-source that stores data in a (semi-) human-readable format, in case the software isn't working. Something cross-platform enough that I can use it on my Linux machine, my Mac, or stick it on a USB stick and access my data from an Internet cafe on the road. A command line UI is fine, and might even be better for portability/compatibility. And most importantly, something I can start using fast, without spending a lot of time learning a new language in detail to write a custom front-end. (MS Access does (HowTos)