Question: I have various devices to connect to my iMac using good-old RS-232.

Of course, I only have USB connectors on my machine.

I have a couple of Prolific PL2303-based cables, and they seem to work OK, but the kext provided seems a little flaky and I'm not sure about long-term support.

What is the most-stable, best-supported USB-Serial chip-set or cable?

Answer: 
I have two Prolific USB-to-serial adapters but they are made by different Vendors (one is Prolific, the other ATEN).

I've used their supplied drivers just fine. Note that there are open source drivers for these devices too available here: <https://github.com/failberg/osx-pl2303> FYI my use case is actually patching them through to a VirtualBox Windows VM and using them in Windows, so I have two layers of drivers and have not had any problems yet.

However, I think the best supported USB-to-serial devices are those made by Belkin. This is subjective, but I've used them for years without problem, and they are a large company with a reputation. They will likely be around in the future.
Just to be clear, **I have not used this device myself.**

Stewart Cheshire, creator of ZeroConf (the basis of Bonjour), gave a Google Tech Talk on the subject of ZeroConf, and presented a couple of embedded devices (Cameras, and an RS-232 unit).

I've always wanted to try this out, but have never had a good enough use to cough up the money to do so.

Stewart presented an RS-232 over Ethernet module from [SitePlayer](http://netmedia.com/siteplayer/telnet/index.html). They have a built in web interface used to set up the ethernet and serial parameters, and if I understand correctly, you simply telnet to the device's IP address in order to be presented with the serial interface. (See their [pdf](http://netmedia.com/siteplayer/telnet/documents/SitePlayer%20Telnet.pdf) on the subject.)