Several freeware antennas design programs.
http://www.qrz.com/download/main/
This is a Java applet which calculates the dimensions and spacings of the elements needed to build a log periodic antenna, given tao, sigma and the lower and upper cutoff frequencies.
http://www.fortunecity.com/skyscraper/jolt/345/LogCalc.html
Antenna Visualization Software
http://n5xu.ae.utexas.edu/antennavis/
Freeware design programs covering virtually everything from dipoles to quagis (quad/yagis).
http://www.qsl.net/ve3sqb/
Freeware.
http://www.raibeam.com/wa7rai.html
Specialized microstrip patch antenna analysis
http://www.antennadesignassociates.com/
http://www.qsl.net/kd4sai/quad1.html
calculate the complex impedance of (short) dipole antenna.
http://www.cetest.nl/zips/selfimp.exe