EZNEC is a well known and often used antenna modeling program. A sample or freeware version is available that will not handle complex antennas - beams, yagis, etc. A full commercial program is also available. The EZNEC web site has several options, downloads and further information.
CW Code Practice
A computer is a perfect tool to practice Morse Code using either the sound cad or PC speaker. Two popular programs are NuMorse and SuperMorse. When properly interfaced, these programs can also be used for CW keying into your rig. NuMorse works best with later Windows version, SuperMorse works best with DOS and older Windows.
CW Code Send/Receive
Morse Code send and receiving programs are another popular PC venue. CW sending is a very simple program concept only requiring a PC-rig interface. CW receiving, on the other hand, is more complex because of all the variations in incoming code levels, speed and signal form. CWTYPE (CW send) and CWGET (CW receive) are two shareware programs that may be used alone or together. Also many of the popular logging programs provide for CW sending from the keyboard.
There is a plethora of digital modes available for amateur radio: PSK31, FT8, RTTY, AMTOR, PACTOR, Packet, APRS - just to name a few. Find popular software packages below.