Xastir is a program for receiving and plotting APRS(tm) 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 FREE! under the GPL license and comes with all source code.