Some added things (in addition to standard first aid stuff) in mine: (BOB anyhow, so only 3 day bag)
ACE bandage (the kind that can stick to itself, these things are just too useful)
A scalpel (and replacement blades)
3 Potassium Iodide tablets (for radiation)
3 Hyrdrocodone (if needed for serious pain)
6 Ibuprofen (if milder pain)
6 Aspirin
3 Bentyl (awesome for any stomach ailment)
1 Vallium (on the chance there's a time when I really may need to just calm the hell down, and am in a safe enough place to do so)
Smelling Salts (ammonia inhalant)
EMT Shears (made to cut through heavy fabric, like denim or seat belts, etc.), same as below quote.
These are the type of scissors I am talking about
I found it online, and it wasn't that expensive, and seems solid enough, did some test cuts great.
The funny part is, I'm the kind of guy who doesn't even usually take an aspirin for a headache (hardly ever even drink), but because my wife has a history of medical issues, I've become pretty aware of different medicines and effects.
I'd rather have them with me, and never need them, than need them and not have them....
_______________________
At home, we have several large box first aid kits (which I've then supplemented with added items): one in each stable (2), one in each bathroom (4), and a kitchen cabinet devoted completely to medicines. I also have a pretty good surgical set, but you have to be dying for me to try that. It's really more for our EMT group member (and she'd bring her own stuff anyhow too).
For non-military types, an IFAK = Improved First Aid Kit, it's the kit issued to soldiers. CLS = Combat Life Saver
Israeli bandages (the best bandage there is IMO)
Those are cool, and you can usually find them online for about $6. I've never seen them in stores.