EZNEC Antenna Software by W7EL
Hah - you beat me to it - I log on this morning to specifically post the same message - EZNEC is now free!
Every ham can easily learn to use this antenna modeling software. How is it useful? Very. I use it to explore "what if" scenarios of existing antennas. For instance, if you have a plain center fed 40 meter dipole at 25 feet, and know and love its performance - what happens to the radiation pattern if you raise one end of the antenna another 10 feet? Another 20 feet? Is it worth the trouble to do that? So easy to do in EZNEC.