PilotWhizz is useful for conversions, though the W&B component is not great for helicopters.
For the US, Foreflight is good for Wx and flight planning (has a link to duats), can e-file flight plans. (Wish you could activate/cancel them in it though!)
Also useful is "Charts" - all US charts, PilotFAR and PilotAIM, once again, for US folks...
I second LogTen Pro.