Method #1, use of accelerometers combined with a known mass of the car. There are a few off the shelf products such as:
http://www.gtechpro.com , I think there is another company making a similar product as well. I'm not really sure its total capability as far as plotting a HP curve but these are getting more sophisticated every year.
#2
http://www.amazon.com/Performance-Ig...0723303&sr=1-1
This book has a step by step procedure to do it with a stopwatch. I highly recommend this book to anyone who frequents this site as there is a ton of good information in this book!
I had #2 in mind when I said it might not be the most accurate.