The knife that spends the most time with me is my EKI SOCFK-A, usually deep pocketed so the sheeple and uniform Nazi's don't spot it...
When not in uniform, an EKI Black Hawk (for my right pocket) and an EKI Commander (lefty model, for my other side).
In PT Gear or sometimes in either of the above situations, an EKI LaGriffe, although I am hoping to upgrade this to "HideAway" in the future.
Why do I carry these? To cut things... fingers and teeth just don't work very well cutting cardboard, rope, opening packages, etc. I like the SOCFK because it is under the assinine 3.5" limit here at USNA and it has the wave. The Black Hawk and Commander have the wave as well... the former is pretty much the "perfect" tactical folder (in my opinion) and the latter is a lefty, so it carries perfectly in my left pocket. The LaGriffe on its ball-chain suspended kydex sheath is convenient, suprisingly fast to deploy (even from inside a shirt), and small enough to almost forget you have it on when PTing, lounging around, sleeping, etc.
Dan