PLIB is a set of libraries to write games and other realtime interactive applications that are 100% portable across a wide range of hardware and operating systems. It's used by Majik3D, FlightGear, and others, and includes libraries for GUI widgets, sound replay, geometry, scene graph, joystick, and fonts/text.
PPTP Client allows you to connect to a PPTP server from a Linux box. It sets up a PPTP call, after which the PPP daemon establishes a PPP link over that PPTP call. The client can access PPTP-based VPNs. Besides remote access to internal corporate networks, some CATV and ADSL ISPs are using PPTP to provide Internet access to their customers.
Seahorse is a Gnome front end for GnuPG, the GNU Privacy Guard program. It is a tool for secure communications and data storage. Data encryption and digital signature creation can easily be performed through a GUI and Key Management operations can easily be carried out through an intuitive interface.
SING stands for 'Send ICMP Nasty Garbage.' It is a tool that sends ICMP packets fully customized from command line. Its main purpose is to replace and complement the ping command, adding certain enhancements as fragmentation; send and receive spoofed packets; send many ICMP information types (echo as the old ping, address mask, timestamp, and router discovery) and errors (redirect, unreach, and time exceeded); and send monster packets. It also supports loose and strict source routing and record routing.