Another vote for Win500, written by the fellow who wrote the firmware for this series of scanners. But any of the three mentioned here will do the job, and it will come down to personal preference. Take advantage of the free trials and decide which one you like best, there is no wrong answer here.
There are no free software programs for the PSR500 and it's later Whistler manufactured scanners. Win500, PSREdit 500, and ARC500 all have thirty day free trial periods, but once that expires, you either have to purchase a license for the program, or discontinue use.