I really like lightweight slipjoints. They are very non-tactical, and easily defensible as a tool, not a weapon. I particularly like wharncliffe and sheepsfoot patterns, since they are especially utilitarian, and non-stabby.
Custom-wise, I've got a T.A. Davison carbon fibre swayback jack. That is the lightest knife I've got. For opening envelopes, cutting tape, and peeling apples, it is perfect. I wouldn't want to pull staples with such a thin blade. I'm sure it would bend, and maybe take a set.
Production, I carry a Spyderco Gayle Bradley Air. It's a lot of blade in a little package. Quick and quiet one-handed opening is a plus. The M4 does stain, but I occasionally clean it with some Brillean, a mildly abrasive food safe cleanser. It has a bit of wax in it. So, there isn't much opportunity for oxidisation. And, when dark spots appear, they scrub off quite easily. The scales are cut a little smaller than the frame. At first, I thought this might be a problem, but the reality is that the little bit of exposed frame provides some more grip in your hand, and keeps the blade from twisting. It hasn't led to any blisters or hot spots.
I'd also be interested in checking out an Al Mar Eagle Ultralight. I think it might be a nice knife to carry at work. It's a little pointy, though, and might send the wrong message.
TC