Wow! A lot more than what I expected to see for recommendations of what "should" be in every photographers bag.
Enough people replied, that I'll post my own list:
1 - camera body. If you're being paid to shoot... then you'll need to make that TWO camera bodies.
2 - standard zoom. I use a 24-70 f/2.8 (full-frame body)
3 - telephoto zoom. I use a 70-200 f/2.8 with image stabilization
4 - at least one fast prime lens (e.g. 50mm f/1.4, etc.)
5 - off-camera flash with a minimum guide number greater than 40 (140+ feet). I carry two of these.
6 - tripod... solid, and appropriately sized.
7 - CPL filter
8 - ND filter (3 stops if you only carry one... or you can carry more than one.)
9 - gray card
10 - incident light meter (one of those things you don't often need to pull out of the bag, but you should carry one for those times when it's the only way to get accurate metering for an important shot.)
I carry a lot more than this list, but to me this represents a basic list of things that anyone who is either pro or just fairly enthusiastic about their hobby should probably have. You can haggle over the camera body and lens choices, but a lot of the non-camera gear are things that, if you don't have them then I think you're under-equipped.