An embedded database. the champagne of beta embedded databases.

Linux for 8086.

• ELKS and Fuzix: Linux – and Unix – writ very, very small @ The Register.

Making Embedded Linux Easy
Buildroot is a simple, efficient and easy-to-use tool to generate embedded Linux systems through cross-compilation.

The Yocto Project. It's not an embedded Linux Distribution,
It creates a custom one for you.
The Yocto Project (YP) is an open source collaboration project that helps developers create custom Linux-based systems regardless of the hardware architecture.
The project provides a flexible set of tools and a space where embedded developers worldwide can share technologies, software stacks, configurations, and best practices that can be used to create tailored Linux images for embedded and IOT devices, or anywhere a customized Linux OS is needed.