USB to Serial Converter
This USB-RS232 converter is based on the PL-2303 chipset and is one of the few with true cross-platform support. Other models are lacking drivers for Mac, Linux, Vista/7, and 64-bit flavors of Windows. If you need something that'll just work no matter what you're running get a USB-RS232 adapter based either on the PL-2303 (like this one) or the FTDI chipsets. If you can't get this to work out of the box, just Google for "PL-2303 <your OS> drivers" and you should be good to go.
TU-S9 Driver Download
We have a half-dozen of these TrendNET TU-S9 adapters in our environment; we use them for connecting to RS232 devices. We selected them because they support both Windows and Macs. They work great.
With the release of Mac OS , the TrendNET adapters no longer work with TrendNET's own drivers. I submitted a help desk ticket to TrendNET's tech support inquiring about the status of drivers for Lion, and received the following response: "We do not have drivers for the MAC OS and do not have any current plans to support the MAC OS."
TU-S9 Driver Download: