XASTIR - is a program for receiving and plotting APRS™ position packets. Development is a collaborative effort of programmers from around the world. Xastir supports many map formats and is highly customizable. Check out the Screen Shots link to see just a few of the ways that Xastir can be used.
Xastir runs on Windows/MacOSX/Linux/FreeBSD/Solaris/Linspire/HPUX, with many more possible. It supports 125 map formats, several types of TNC's and weather stations, Festival speech synthesizer, AX.25 networking (Soundmodem/Baycom/SCC!) and seven languages!. On Unix-like operating systems, you can run your weather station or GPS on a separate computer as well, which is good for those hams who are already short on serial ports.
Xastir is one of several APRS programs that support SmartBeaconing and CornerPegging.
Xastir is FREE! under the GPL license and comes with all source code.