I ride a lot on trails where there are dog walkers, trail runners, families with kids, etc. As well as other mountain bikers. I have a bell on my bike, and when I want to let someone know, in a friendly way, that I'm approaching, I ring it. It saves my voice from screaming "on your left" constantly, it's audible over a long-ish distance, and it cost $4 to put on my bike.
You would think this accessory would be common among the sort of person that is perfectly willing to spend several thousand dollars on a bicycle. But you would be wrong. I'd estimate 95% of the people I see riding bikes on the trails around here would prefer one of the following to installing/using a bell:
-The Stealth Pass. Maintain total silence as you approach the unsuspecting pedestrian, relying soley on their innate 6th sense to warn them to move out of your way. Act really peeved when they don't move quickly enough.
-Ride off the trail. Who cares about singletrack? If you want to save 10 seconds getting back to your car, you can just ride around the silly hikers off the trail (yes, you can combine this with other techniques)!
-The triathalete scream. You all know it - "On your left!" Unfortunately, sometimes there *isn't* a good way for someone to get out of the way, yelling is pretty unfriendly, and it's hard to yell at someone from 50 feet away without losing your voice after a few repetitions.
-The inappropriate pass. You're 10 feet from the trailhead, finishing your ride. Is this really the time to rail around someone? YES! Alternately, a line of 10 riders is coming uphill - should you yield to them, ruining your shredtacular descent? Naw! If they don't get out of the way, just ride off the trail.
If you are riding on crowded trails, and you don't have a bell, you are basically an idiot, in my book. It's also possible, depending on what passing technique you use, that you're ruining the trail, endangering bike access, or even risking hurting someone (possibly yourself).
So get a bell. If you can't swing the $4, stop by Waltworks World HQ and I will give you one for free and help you install it. No joke. Then ring away and make friends out there, while proving to everyone that you are not a moron.