Most modern Ham Radio Rigs provide a CAT (Computer Assisted Tuning) interface which offer the ability to control the radio from a computer.

A good number of CAT interfaces are based on protocols layered on the standard serial interface.

• ICOM CI-V CAT - 3.5mm mono jack, half duplex, open collector serial interface - transmit and receive connected to a single line.
• Yaesu CT62 CAT (ACC connector) - 8 pin mini DIN, full duplex, 5V serial interface with 13.8V output

Cables including a USB to serial converter are readily available on ebay for about £9 including shipping:

Both cables are based on the Prolific PL2303 chipset - drivers for both Windows and Unix available on the Prolific website Prolific