Most people don't see utility in a fixed-blade knife (a.k.a. sheath knife, hunting knife, camping knife, and even survival knife and, wait for it, combat knife). Those who do are usually nuts who carry huge and very mean-looking chunks of steel of poor quality and dubious purpose.
A fixed-blade knife is, in fact, quite useful. A personal story: On a mountain trip, when we going down a valley after three weeks with no contact with civilization, we discovered that the valley was not what we expected it to be: during preparation for the trip we discovered no record of anyone ever going through it, the only hand-driven map of the locale we had access to had white space with a black line for the stream, and satellite pictures were hard to decypher (it was back in pre-www days when you couldn't get high-quality satellite pictures of any area in seconds, so we had nominally 1-meter resolution pictures declassified in Russia that were, in fact, quite poor). We assumed that we would go 40km (or was it 30?) down the valley in two days. When we got there, we made it about 5km down in four days and the prospects were just as bleak from there: the valley was passable, you just had to start your day hiking 800-1200m (2600-4000 feet) up and finish it by hiking same elevation down back to water so that you could camp; the part on top was passable, but it was taking us a long time. In fact, the date on our return train tickets was approaching and our food was beginning to run low. There we sit, having descended to water already in the dark, longingly looking at the opposite side of the unpassable stream, trying to decide if we should go back to the glacier and cross where we can, losing a week (for which we had no food) or try to cross. We started throwing an ice axe on a rope to the other side, trying to make it stick and trying the other usual tricks; they would not work. So, I chose a pine high enough to reach the other side (it was perhaps 30cm in diameter or 37 inches in circumference for the SI-challenged crowd), borrowed a fixed-blade knife (rather average quality, light and short) from the only person who packed one (we had no wood axe, as that is heavy and most of our trip was not in a wooded area: we carried our chemical fuel tablets with us), put my rope gloves on and started nibbling at the tree. Everyone, of course, laughed at my idea of felling an old-growth pine with a knife and went to sleep in the tent. It took me maybe about 10 hours to fell the tree and it felt good: no sores or anything. Once the tree was down, another person crossed the stream on it (crampons, harness with a 10mm rope tied around another tree on our side and four men holding, no backpack) and hung a rope on which we slided to the other side; we were down at a shepherds' summer camp in another day and a half---with no food left. This episode convinced me that a fixed-blade knife can be used in place of most other tools.
Blade length: 10-20cm (4-8 inches), depending on primary uses. If not sure, probably best to get a smaller blade. If you plan to normally carry a wood axe in addition to the knife, be sure to get a smaller blade.
Blade thickness: 3-6mm (1/8 to 1/4 inch), depending on length.
Blade style: drop-point (stronger than clip-point).
Full-tang or encapsulated-tang handle (check if suitable for hammering), non-serrated blade, protective coating good, high-carbon steel better than stainless steel (unless primary use is related to salty water), industrial steel (such as A2) good, for larger knives a comfortable finger groove in the base of the blade (a choil), textured handle probably better than smooth, a reasonable handguard (no ``fighting''-style silliness, though), a hole at end of handle to put a loop of parachute cord through might be useful.
Sheath: Kydex is a plastic material that retains its shape without loosening up for a very long time. A kydex insert in a heavy nylon stuff is quite convenient. For a smaller knife a leather sheath might work just as well.
There are certainly many other reputable knife makers, but these are probably the most recognized. Be sure to consider Himalayan Imports---khukuris (very tempting, some made by the Nepalese Royal smith). The khukuri (also spelled kukri, khukri, kukhri) is a knife traditionally used by Nepalese Gurkhas both as a universal tool and a weapon. Because of an appallingly low standard of living in Nepal the knives are dirt-cheap. Keep in mind that they are forged by hand, with small variations and little imperfections. Just like the knives above they come with lifetime no-nonsense warranty. One consideration for me was that the state of Michigan excludes hunting knives from its regulations, and the khukuri is unlikely to be classified as one. I ended up buying one, made by the Royal Kami, to admire and to brag about, but I don't actually know how to use it.
I have bought a Busse Steel Heart Ergo. It is probably the most rugged object I own. I love it. After playing with it for a while, the only similar size knives that I remember with craving are the ones I have seen in Japan in swordsmiths' workshops, but those start at $2000 (before you ask, a katana is $15-25k: it takes a very long time to make them by hand and they are made by very highly skilled masters in an industrialized country with a high standard of living and an export-oriented economy).
I do think that a locking blade is quite a bit safer even if one is only going to peel oranges with it. Wenger makes several locking knives in traditional size (~3.5"). I like to have most basic stuff that I would need handy: scissors, tweezers, a corkscrew, and a bottle opener. There are two models that have these: Survivor and Nomad. The Survivor is quite a bit larger, includes a poor-quality but fat whistle and a lot of useless (to me) junk, and has a fully serrated main blade. I prefer a non-serrated blade because it's more versatile, easier to sharpen, and looks more traditional and therefore attracts less adverse attention. Wenger Nomad 16988 has a main locking blade that is non-serrated and the model still includes what I want while being quite a bit smaller than the Survivor. A winner (even though it has implements I don't want, such as the small blade and a universal wrench)! It's also cheaper ($30-35). (Wenger Nomad is the only 3.5" SAK that I could find with a non-serrated locking blade.) If you're OK with a partially serrated blade (presumably you have the tools and patience to sharpen it) and don't need a corkscrew (poor fellow!), you could get Wenger Adirondack: it's even thinner and cheaper.
The Nomad model could be improved by losing less useful implements to reduce the number of slots from four to two: one middle slot is occupied by a locking Phillips screwdriver and a universal hex wrench (both of these can go); the other has cap lifter and can opener (can be replaced by a single combination cap lifter/can opener/screwdriver tool that Wenger has and the tool can go in place of the small pen blade that seems to serve no useful purpose in the presence of the very good main locking blade). This would make the knife twice thinner (and probably cheaper, too) while almost not reducing its usefulness. One slot would contain on the face a large locking blade and a combination cap liften/can opener while the other would have the scissors; the back would have a corkscrew on one side and a reamer on the other. Too bad Wenger doesn't have a lighter model like this.
If you're OK with occasionally cutting your index finger to the bone while trying to pry a walnut open with the blade, you can probably easily find a SAK that has the exact set of attributes you want; it would have the advantage of not needing to be suspended from your belt in California. If you live in California and want a safer knife with a locking blade, you'll probably have to get one of the belt knife pouches that both Wenger and Victorinox make.
