At the weekend, it's just the standard phone, wallet, and keys, although I also have an annoying habit of stuffing receipts in my back pocket so they pile up a bit there.
On weekdays, I take a backpack for questing which usually contains:
Loads (i.e. several kilos worth) of print-outs of papers, handouts, and other stuff I may need
A couple of notebooks
Some library books
A stapler (currently missing!)
An umbrella (occassionally, although I usually just forget and just get wet)
blue pens
a pencil
green and red biros
whiteboard markers
USB stick
earphones
reading glasses
hand moisturiser in winter, or my hands get horrendously chapped
dental floss (because there's nothing more annoying than having something stuck in your teeth and not being able to get it out all day)
tissues (especially when I have a cold)
Do I win heaviest bag?