My keys, mobile, purse (which has all my money, my debit card, oyster card and passport - better not lose that!) are the main 3 things I just HAVE to have with me. My makeup bag if it's going to be a long day or I'm going out anywhere special. Plus, usually a bottle of water because I get sooo thirsty.
I have friends who take the whole kitchen sink with them. It can be useful though. Generally, I can ask them for something and they WILL have it.