Six languages supported: English, Spanish, German, Dutch, Italian, and French. Portuguese rumored to be supported soon.
Real-time tracking of objects via RF or internet feeds.
Understands approximately 114 different map formats, including the ability to tile USGS geoTIFF images together into a contiguous image. Can use online data sources as well: Terraserver, Tiger, weather radar images, etc. Many vector and raster map formats supported: Shapefiles, pocketAPRS maps, Win/DOSaprs maps, ...
Runs on Solaris, FreeBSD, Linux, and Mac OS X so far.
Supports Control-E for Kantronics and Paccomm TNC's that have that feature. GPS & Radio connect to the TNC while the TNC connects to the computer via one serial port.
Supports regular serial-port TNC's as well as kernel-mode AX.25 drivers, which give Xastir the capability to use SoundModem drivers, KISS-mode TNC's, SCC cards, Baycom TNC's, etc.
Speech capability with Festival support.
Many more features! See the README.1ST file and the help files for more details.
Oh yea, did I mention that you can download the sources for free? It's under the GPL license.