Alexander Wolf

It's good and lightweight planetarium for UNIX/Linux systems at the start of life (also available for OS X and Windows now). It has few powerful tools for advanced users but has some lacks in visualization of the celestial sphere. KStars is part of KDE and this planetarium are in active development - many contributors are participants of Google's contests - Summer of Code and Code-In.

Sky Chart (also known as Cartes du Ciel) is very powerful planetarium for advanced astronomy amateurs and professionals-astronomers. Supports of many catalogues are made it very popular. This planetarium has active usage for management of the mounts of telescopes through ASCOM.