So there's three things I don't leave the house without. All very practical really.
1) My phone
My phone always goes everywhere with me. It's ultra handy now I have a Blackberry because I can keep track of my emails when I'm travelling between home and uni (for example) meaning I can email contributors for the uni magazine or email my tutors a sudden, pressing question regarding an essay.
2) My purse
Unless I'm so hideously poor that I don't need it of course. This beauty was given to me by my grandma a while back - at least a year and a half ago - and is brilliant for storing all the junk I like having in my purse. Examples of said 'junk' include the mini statements you get when you withdraw cash, deal cards for an amazing hot dog stand back home, my Waterstones card and the cards taxi ranks give out (because I always forget to store them on my phone!) Also I tend to have big bags so a giant purse like this makes it easier to locate when I'm going to pay for something.
3) My keys
Again, another obvious one. No point leaving if you can't get back in. My keys are ridiculously big for the amount of keys I have. I have *counts them* three, four if you include the padlock one I have for the kitchen cupboard (food thief) and a whole bunch of keyrings. Admittedly, as with my purse, this makes them easier to locate. Each key ring means something to me. I also have one of those trolley coins (in lei of an actual pound coin) with Bagpuss on it, which comes in handy more than I expected it to.
And tah-dah! Those are the things I will have on me (or in one of my giant bags) if you bump into me on the street.