Equivalent to one egg:
1 tbsp ground flax seed*
3 tbsp water
Whisk together in a shallow bowl with a fork. Add to recipe as usual.
*I have a spare coffee grinder in my spice cupboard to grind things like this.
Extra tip: You can clean your coffee grinder by grinding plain white rice in it. It will will clean the sides and blade. For cleaning something that was very fragrant, grind two batches of rice. It should come very clean.