I carry a Cold Steel Ti-Lite VI with a 6" Blade.

It's basically a modern take on the classic Stiletto style folding knives, although unlike most others, this one isn't spring-assisted. It comes open easy (after a short break in period) with just a push of the thumb and a flick of the wrist.
Being a Stiletto spear point blade, it's better suited for self-defense than a general purpose knife, although it is still razor sharp and can be used for more, it's designed as a stabbing weapon rather than a multi-purpose cutting tool.

For a multi-tool, I reach for the smaller, more convenient Victorinox Swiss Army Knife.

What can I say? I'm a sucker for the classics.