What is maemo? Wikipedia explains, Maemo is a software platform developed by Nokia for smartphones and Internet Tablets. It is based on the Debian Linux distribution.

The platform comprises the Maemo operating system and the Maemo SDK

Maemo is mostly based on open source code, and has been developed by Maemo Devices within Nokia in collaboration with many open source projects such as the Linux kernel, Debian, and GNOME. Maemo is based on Debian GNU/Linux and draws much of its GUI, frameworks, and libraries from the GNOME project. It uses the Matchbox window manager, and the GTK-based Hildon as its GUI and application framework.

The UI in Maemo 4 is similar to many handheld interfaces, and features a "Home" screen, which acts as a central point from which all applications and settings are accessed. The Home Screen is divided into areas for launching applications, a menu bar, and a large customisable area that can display information such as an RSS reader, Internet radio player, and Google search box.

The Maemo 5 UI is slightly different: The menu bar and info area are consolidated to the top of the display, and the four desktops can be customized with shortcuts and widgets.
At the Mobile World Congress in February 2010, it was announced that the Maemo project would be merging with Moblin to create the MeeGo mobile software platform.

And now maemo already implemented on the Nokia N900 (see previous post)