I carry a Gerber Applegate-Fairbairn "Mini-covert" folder. It's pretty small, but durable, and light. I think the blade is something like 2.75 or 3.00 inches. I trust the Gerber brand pretty well, and the knife is still tight and solid after a year's wear. Only cost me about $20 on eBay.
I've got a couple other decent, low-cost knives that I would carry if the Gerber malf'd.
First choice is a CRKT m16-10kz (or whatever crazy alphanumeric it is). It's a tanto folder, not dissimilar to the Gerber. Only real downside is the locking mechanism, which has the "Autolawks" thingy; a lever secondary to the liner-lock that has to be manipulated in concert with the liner lock to close the knife. Meaning that I have to shift grip to close the blade, but it's still doable with one hand.
I also have a Kershaw Vapor that's very solid, with slightly more heft (metal handle). I'm just not crazy about the blade shape, but it's a good knife.
I bought my friend a Beretta folder some years ago as a birthday present. It's got a slight backward curve at the tip, which he likes, and it's seen harder use than I give my knives without problems.
One thing to consider is what your area's knife laws are. In California, where I am, I can carry a non-assisted opening folder legally. More importantly, I suspect, is that it's not something that a police officer is likely to come down hard on, legal or not. Fixed-blade knives, even though I think they're legal to open carry here (but illegal to conceal), will almost certainly get unwanted attention from someone wearing a badge.
Some people believe that using a knife requires extensive training. I only partially agree. Training helps, but a knife can be used on whatever level you need to use it on. Sometimes that's as simple as "Pointy end goes into the other guy". People used knives to kill each other long before they formalized the method to do so. And I have more instinctive trust in the cold steel of a knife than in a chemical spray or stun gun.
YMMV, I'm not an expert at anything, and frequently don't know what I'm talking about, (insert additional disclaimer here).