Generic black JanSport daypack. I've tried a bunch of others over the years, ended up returning them all.
This is essentially the same backpack I used in high school/college.
They're better now: the back and straps are padded, there are more pockets and they are more useful (like the mesh pocket for water bottles). Even things like the zippers will converge at one point so I can use a small padlock to lock almost everything up.
That's what I take to work and travel with. If I'm just going out, I don't need it. Keys, wallet, cellphone.
Today's smartphones make lightweight traveling easier. I don't need to carry around the following: camera (plus extra rolls of film), notepad, maps, train schedule, pen, tourist guidebook, GPS, music player,
If you carry around a notebook computer, I highly suggest getting a neoprene sleeve and not getting a notebook backpack with a dedicated padded pocket. The standalone sleeve is more versatile, as well as the backpack (for times when you aren't hauling a computer).