How loud are gunshots?

Most modern firearms are somewhere between 150-170 decibels depending on several factors such as caliber, barrel length etc.
Here's a page with decibel readings for numerous configurations:
http://forum.pafoa.org/showthread.php?t=106813

To put that in perspective here are the typical decible levels of other common "loud" sounds:

30 db = quite room at night
60 db = average conversation
70 db = vacuum cleaner 3 feet away
80 db = busy road (standing on the sidewalk a few yards from traffic)
90 db = Semi Truck engine from 10 yards away
100 db = busy night club with moderate music volume
110 db = gas chain saw
120 db = loud enough to cause physical discomfort and permanent damage no matter how short the exposure time
150 db = Jet aircraft from 50 yards away

Bottom line - gunshots are loud. For many people gunshots will be the loudest sound they ever experience. That's why any sane person will use hearing protection when shooting, or even when they're near other people who are shooting. Many people use both ear plugs AND ear muffs when shooting. If you shoot regularly, you'll want high quality ear muffs with more sound dampening effect than the basic muffs you might wear when using a lawn mower or chainsaw.

I found this list
http://www.m1911.org/loudness.htm

I can't add to David and Stephanie's answer. Tells it all.

Loud enough to make you go def temporarily, and over extended times can cause permanent hearing loss. Always wear hearing protection when shooting.

A loud "clap" of two bricks, then amplify it by two...that should be accurate. Other factors to take into account is barrell size, caliber, the grain value of the ammo, and weapon manufacturer.

Imagine the impact of two very large wet breasts being slapped repeatedly against the side of your head like the force of a full grown seal slapping its tail. At least that's what I think about when I'm shooting.