This list was from another thread but thought others may have their own ideas to add.
Things I lie to my kids about:
Their pictures are brilliant
Father Christmas
The truth of the Holocaut*
The toilet paper with blood on*
That David Beckham does wear his vest when his mummy tells him too
What really happended in the Tsumani*
What child abuse really is*
That he's the cleverest boy in the world
That I'm really a Power Ranger who goes out at night to fight evil
The tooth fairy
That he is bound to be able to play for West Brom when a grown up
What happened in Rwanda*
What mummy's vibrator is really for
That he can't have a playstation until he's 16
That Mummies really can stop you joining the army even when you're grown up
The hellish truth of war*
The ones with an * I intend to provide the truth at a later date, as they are important. Others they will work out themselves.