Hmmm, software for free, what a concept. Sorry to disappoint you, but this billing software is becoming a BIG business! At $600-$1,000 per program, for a good one, these companies are just starting to make some money. In the long run, however, I think that a billing program for under a thousand dollars is a bargain! Most people who convert from paper billing to software make back that much money and more in a very short time by simply tracking their accounts receivable.
As for assessment measures,try MetriTech's book Psychware Sourcebook (Krug). I believe that this book lists a few of these programs!
lr