Mer is a new operating system for small, mobile touch-screen devices.
It is Linux based and layers the best open-source elements of Nokia's Maemo platform over a modern Ubuntu base.
The goals of Mer include:
Integrate the best solutions for a wide variety of small form-factor devices
Encourage wider access to device capabilities through the Vendor Social Contract
Demonstrably provide an easy route to market for vendors
Dramatically reduce costs to vendors of supporting EOL hardware
Focus, harness and support community contributions to the platform
Encourage and ease migration of existing applications
Support experimentation, innovation and development
So basically Mer is Maemo-platform on top of Ubuntu. This means Maemo's finger-friendly and innovative user-interface and good powersaving features with Ubuntu's software! Currently Mer looks like this. But version 0.17 is supposed to get the new, really cool Maemo 5 UI. Well, not exactly as seen on the n900, but all the open-source components available
Now Mer'd be cool on Touch Book, wouldn't it? It's running on top of Ubuntu, so most of the Ubuntu software is working on it. It's not ready yet, but i really like it on my n810. Touch Book has faster processor and graphics..
Installation how-to here (once there is Mer Touch Book images, there'll be better instructions in the Mer wiki.)