Definitely a multitool like a leatherman. I wish there was a check-in just for multitools on a plane since thats the only thing I can't bring with a carry-on. Obtaining a quality multitool in some countries can be hard/expensive.
I carry my Leatherman everywhere, and it's helped me more times than I can count. Maybe it's just me, but I feel better knowing that I have a pretty complete toolkit on me at all times.
And no, it's not a security risk. In the post-9/11 world, passengers know that cooperating with hijackers is a potential death sentence and will fight for their lives. Cockpit doors are kept locked.
A group of people armed with pocketknives isn't going to be able to do much.