I just purchased this one by Startech. I am primarily running this on ubuntu and it has been running without a hiccup. I've also used it on win7 without fail. This was the first FTDI chipset adapter that I purchased and I must say it works LOADS better than the Prolific pl2302 chipsets that tend to fail, crash, or constantly lag behind depending on if your system is 64 bit or not. I could just be unlucky but i've had 10+ different models based on the Prolific chipsets and every single one of them eventually crashes the computer. With the FTDI chipset I haven't had a single problem, I'll never get anything but FTDI from now on.
If you want something a little more multifunctional, I've also recently purchased this and have it connected to a Linux based server. Plug-and-play and it works flawlessly as well. Also based on FTDI. The only problem with this is that each of the 4 serial connectors have male screws. The issue with this is the cisco cables are also male screws. I had to buy some female stud bolts for a serial port to install on each of the ends and now my cables don't randomly fall out!
The two units I see consistently good performance with are:
The Keyspan uses a somewhat unique Texas Instruments chipset while the Trendnet uses a Prolific chipset.
I hear lots of people complain about Prolific chips and fawn over FTDI chips, but I've never had a noteworthy issue with a Prolific chip device.
I've never used this one, but if you want an FTDI adapter this one should be good: