Dzen is described as 'general purpose messaging, notification and menuing program for X11. It was designed to be scriptable in any language and integrate well with window managers like dwm, wmii and xmonad though it will work with any window manager' and is an app in the OS & Utilities category. There are eight alternatives to Dzen for Linux, BSD, Xfce and Mac. The best alternative is Cairo-Dock, which is both free and Open Source. Other great apps like Dzen are tint2 (Free, Open Source), Avant Window Navigator (Free Personal, Open Source), Dock (Paid) and ADeskBar (Free, Open Source).
Avant Window Navigator (AWN/Awn) is a dock-like navigation bar for the linux desktop that positions itself at the bottom of the screen. It can be used to keep track of open windows and behaves like a normal window list.
In OS X, the Dock is used as a repository for any program or file in the operating system. It can hold any number of items and resizes them dynamically to fit while using magnification to clarify smaller resized items.
PyPanel is a lightweight panel/taskbar written in Python and C for X11 window managers. It can be easily customized to match any desktop theme or taste. PyPanel works with EWMH compliant WMs (Openbox, PekWM, FVWM, etc.) and is distributed under the GNU General Public License v2.
BMPanel (bitmap panel) is a lightweight, NETWM compliant panel for X11 Window System, which contains a desktop switcher, taskbar, system tray and clock. The application is inspired by simplicity of fspanel. BMPanel has a modern look and feel, while keeping itself tiny and small.