2 smallish panniers and a biggish handlebar bag (which I should downsize really; it's turning into a purse...) to hold work clothes, rain gear, lunch, U-lock, small tool kit (multi-tool, 1 wrench that fits my oddball Raleigh, tire levers, patch kit, tube, Swiss Army knife), small first aid kit, cell phone, baby wipes, water bottle (I need a bottle cage!). I could pack lighter if I left clothes at work, but I'm afraid if I did that I'd end up with no pants one day.
Probably about 10-15 lbs altogether, but considering I ride either an old touring Raleigh or a Norco mountain bike that has been dubbed "The Tank," it's not a lot in the grand scheme of things.