I recommend the StarTech 1-Port USB to RS232 DB9 Male to Male Serial Adapter Cable (ICUSB232V2). It has a 3 foot cable and uses the FTDI chipset.

Claimed as easy to use, the E5805A USB-to-RS232 interface can connect up to four RS232 test instruments to the USB port on a personal computer at baud rates up to 230 Kb/s per serial port.

If you’re attached to that favorite DB-9 interfaced device you should look into this part. FTDI is selling a USB-RS232 adapter as a replacement for DB-9 connectors. They come with USB male or… ...