alttab provides convenient visual interface and switching on releasing Alt, not on pressing Tab key. Being lightweight and depending only on basic X11 libs, it still features customizable look and behaviour.

ckb-next is an open-source driver for Corsair keyboards and mice. It aims to bring the features of Corsair's proprietary CUE software to the Linux and Mac operating systems.

XTide is a package that provides tide and current predictions in a wide variety of formats. This package enables the xtide package to display the world's coastlines (World Vector Shoreline data) when displaying the Earth to select tidal stations.

Nice graphical 'df', showing you to what degree the mounted filesystems are used.

This tool is run by a nightly cron job, and takes a `diff' of specified system configuration files and emails them to a specified email address. Options to diff can be specified. This is useful in friendly environments where there are multiple sysadmins working on configuration files/setups and everyone's changes are reported. CVS is a better answer, but the most of people don't want to have the hassle of putting system files in a repository.