I don't carry a flight bag anymore... we just leave everything on the plane (another benefit of the corporate life)...
But here's what I carried when I worked for the airlines:
Headset (in tupperware case)
Calculator
Two Jepp binders with company plates
Two Jepp map cases
Extra small binder with jepp sized "handouts"
-ALPA duty time/reserve rules
-Contract
-JSAP/ASAP program forms and info
-WX Radar book
-Company airport information pages
Company Flight Standards Manual
Company Flight Operations Manual
2 D cell Mag Light (for whomping)
2 lithium battery small 6" halon flashlight (for preflights)
Captains Atlas
Aluminum Can with:
-extra pens,
-highlighter,
-paper hole fixer things,
-a small stapler,
-Aircraft Circuit Breaker list/Mechanical Malfunction Cheat Sheet,
-spare checklist,
-spare weight and balance forms,
-manual dispatch forms, and
-AM radio station list.