The Prolific PL-2303 chip seems to be a quite popular solution for USB to (classic) serial converters - in fact, all of the products I have seen until now use it. Just not all the manufacturers include the MacOS X driver.

A new version of the PL2303 MacOS X driver is now available from the Prolific support center site (as of 20060208, look for the file named md_pl2303H_HX_X_dmg_v1.1.0b1.zip).

The following info is obsolete now a new driver is available, and is just kept for reference:

It used to be available from the Prolific tech support site, but that site seems to be currently offline. For that reason, a copy of the v1.06 OSX driver I found on the net is temporarily available here: ProlificUSBSerialOSX106.zip

The driver works with Panther (MacOS X 10.3), but regularly generates error messages like the following on the console on some machines:

mach_kernel: ttyioss800000?: dequeueData ret 3
mach_kernel: 00000?: dequeueData ret 3

Fortunately, someone at macOSXhints created a patch (cache) for the driver binary that removes the messages. Instead of downloading a graphical hex editor, you probably want to use this method (cache) for the actual patch step.

The procedure boils down to something like this:
(become root, unload driver, change to kext dir, make a backup copy, make file writable, apply binary patch, restore permissions, reload driver)

Warning: this worked for me on my system with the driver available from this page, but all sorts of things may go wrong on yours.
 > su -
 # kextunload /System/Library/Extensions/ProlificUsbSerial.kext
 # cd /System/Library/Extensions/ProlificUsbSerial.kext/Contents/MacOS
 # cp ProlificUsbSerial ProlificUsbSerial.old
 # chmod 0777 ProlificUsbSerial
 # printf "\070\140\000\000" | dd bs=4 seek=2210 count=1 conv=notrunc of=ProlificUsbSerial
 # chmod 0555 ProlificUsbSerial
 # kextload -v /System/Library/Extensions/ProlificUsbSerial.kext
 # exit

If root access on your system is not enabled, look at the EnablingRootOnMacOSX page.