www.duats.com
We use DUATS for flight planning for our 200 King Air. You can't beat the price (free), and web access makes it easy to get weather, flight plan, and file even from the road. I have tweaked my numbers in my performance profile, and my enroute times are always +/- 3 minutes of actual. The fuel burn isn't the greatest, since it won't compensate at all for altitude. At low alts, I'm burning 700PPH or so, but in the upper 20s I'm down around 500. Big difference. I keep my fuel numbers set to pessimistic values and always come in under planned fuel consumption, which is the way I like it.
DUATS will also let you file ICAO flight plans, which is nice. We go to the Bahamas and Canada occasionally, and DUATS does good work there.
I have tried out fltplan.com, and I think the performance numbers (especially if you customize them, but that is a lengthy process) are more accurate. However, its database seems more limited. I tried to run a Nassau flight plan on there recently and it doesnt recognize any of the BR routes, which DUATS does.
R