The SignaLink USB
The SignaLink USB is a small radio interface. It appears to your computer as a USB sound card. It has a completely isolated radio interface and is sold in various configurations with prebuilt cables for many popular rigs. It's less than $100, which means it's very popular.
The SignaLink USB can key the PTT circuit on the rig and has an internal VOX circuit for doing so, based on the transmission from the computer. It with Windows and Linux.
The SignaLink USB is sold by Tigertronics. Links:
The SLUSB is designed for digital transmissions. I wanted to use it for voice transmissions as well, such as for my CQ call during SSB contests. The trouble there is that the VOX circuit is designed for digital, and will rapidly drop PTT and re-engage it during a voice TX. Depending on what you're doing, turning up the VOX delay by using the delay knob may help. But I decided that I wanted something more reliable.
My first solution to this was to use audacity. I could create a 44100Hz 16-bit WAV, mix in a 21000Hz tone, and this would work fine. The tone would keep VOX keyed, but yet will be filtered out by the radio and won't be heard on the air.
I wanted to automated this further, so I made some Linux scripts for it.
See LinuxSignaLinkUSBVoice for details on how I did it.