skiTunes is a fast, extensible alternative to iTunes. It can use Apple Quicktime, VLC, MPlayer, LAME, and cdrecord. It differs from Apple's offering in that it is written in Cocoa and uses native widgets, as well as supporting formats iTunes doesn't, such as OGG and WMA. Using native widgets will allow it to automatically match the rest of the OS when a theme is applied, unlike iTunes, which requires hacking in order to make it match. It will also offer a skinnable "mini- mode".
skill sends signals to processes given any combination of user names, ttys, commands, and pids. snice changes the priority of processes (given the same). They are similar to kill(1) and renice(8), but the commandline is order-independent. There are also verbose, search, and interactive modes of operation.
skipfish is a high-performance, easy, and sophisticated Web application security testing tool. It features a single-threaded multiplexing HTTP stack, heuristic detection of obscure Web frameworks, and advanced, differential security checks capable of detecting blind injection vulnerabilities, stored XSS, and so forth.
skyviewer is an OpenGL-based program to display HEALPix-based skymaps, saved in FITS format files. The loaded skymaps can be viewed either on a 3D sphere or as a Mollweide projection. In either case, realtime panning and zooming are supported, along with rotations for the 3D sphere view, assuming you have a strong enough graphics card.
Slash'EM (Super Lotsa Added Stuff Hack -- Extended Magic) is a variant of the hugely popular roguelike game NetHack. It adds more monsters, more objects, more dungeon levels, more roles, and more races, shopkeeper services, and techniques for you to use, and invisible objects and gypsies for those who feel lucky. It also comes with a GTK windowing interface and higher resolution tile sets as options.
Slidematch is a media rating, contextual retrieval, peer recommendation, and library management system for music, video, movie, and book impressions. It adds value to your existing media player by enabling you to easily record detailed impressions, retrieve content appropriate to what you are currently doing, and interact with friends.