I use one of these on three different computers here...
USB to TWO RS-232 Serial adapter ES-U-1002-M-NS + cable | eBay
Its on my feed computer for tags and programming, on my main desktop for serial applications such as digital modes on HF, and i use one on the laptop for programming radios as well. Its the only one i have found that will do everything i throw at it. Any of the FTDI chipset based models such as this are a good buy, they act just like a hardware port and nothing knows the difference.
It also has a 5v output on it that can be enabled by an internal jumper, works great for running a GPS or APRS tracker as a KISS tnc on it as well.