By using a special adaptor, you can connect your serial-line UPS to a USB port. If you would like to free up your serial port and connect your existing serial port UPS to a USB port, it is possible if you have one of the later kernels. You simply get a serial to USB adapter that is supported by the kernel, plug it in and make one minor change to your apcupsd.conf file and away you go. (Kern adds: Thanks to Joe Acosta for pointing this out to me.)
The device that Joe Acosta and Kern are using is IOgear GUC232A USB 2 serial adapter. Bill Marr informs us that it also works with a Back-UPS Pro 650 and the 940-0095B cable.
At Kern's site, running Red Hat 7.1 with kernel 2.4.9-12, he simply changed his /etc/apcupsd/apcupsd.conf configuration line to be:
DEVICE /dev/ttyUSB0
Depending on whether or not you have hotplug working, you may need to explicitly load the kernel modules usbserial and pl2303. In Kern's case, this was not necessary.