This is a question we get fairly often, and unfortunately standard converters will not work. The reason is that both Mach 3 and EMC2 depend not only on the signals coming out of the parallel port, but also on the timing of those signals to control the motors. These programs are operating in an effectively "real time" environment to make sure that motions happen at the right speed from multiple axes. While USB is a much faster and newer protocol, USB converters do not maintain the timing associated with the step and direction signals, rendering them useless for CNC. The parallel port is really one of the only interfaces on PC's that allows for the level of direct control required for CNC.
There are a couple of options. The easiest and cheapest is usually to find an inexpensive used desktop computer with a parallel port -- these can often be found for $60 or less. Indeed, due to the level of control Mach 3 needs over the PC, we recommend having a separate computer for machine control from the one you use for CAD / CAM work. The second option is to install an EPP compatible parallel port PCI card in your computer. These can be found for under $20 online.
Lastly, there are a couple of companies that offer USB card products with their own plugin to Mach 3 -- these offload the realtime requirements from Mach to a DSP chip on the card. The are pricier than either a parallel port or a used PC, but they do offer reliable signals and faster pulse rates than a parallel port can typically offer. Check out both SmoothStepper and Dynomotion for more information.