I use aeroplanner.com - it's the best by far (and I've tried a lot).
DUATS is free to use in the US (providing you have an FAA pilot/medical cert. to access it), and has some basic tools, but no advanced planning (more for Flight plan filing/weather, etc).
aeroplanner has four "tiers" - free, basic, premium, plus. I have the premium version (paid for), as it also acts as my electronic log book, but even the free access version allows basic planning. The paid versions allow LOADS of cool things - including the ability to print out EVERYTHING you need for a full cross country trip (even the sectional chart extracts!)
Go to the website -
www.aeroplanner.com and see what you think. Personally, it took me about 3/4 different sites to settle on that one, and it's done me well for US flying over the past year or two.
If you have any questions, drop me a line.