http://www.comtrol.com/sales/specs/rpusbsh.htm

This is the one I use. It's a RocketPort USB Serial Hub. It's got 4 serial ports. So I have Com5, Com6, Com7 and Com8 on my computer. It's very useful!! :) An the Com5 through 8 are non interupt based so you don't have to worry about interrupts. It's very compatible. I highly recommend it. It costs more though.

Calvin