Developer: Avantar LLC
Price: free Download on the App Store
With the web-browsing abilities of the iPhone, it's generally pretty easy to find a phone number; a little Googling will usually get you what you need. That being said, Yellow Pages by Avantar conveniently makes it even easier to find whatever it is you're looking for. For example, I did a search for live music in Tulsa, OK to get the results in the screencap to the left. Handy, right?
The controls for Yellow Pages are very simple and pretty much exactly what you'd expect. You can search for a specific business, or simply, for example, search for dentists in a particular area. Yellow Pages uses your iPhone's GPS feature to help narrow search results to your current location. However, you can also easily modify your search to a different location.
The home screen for Yellow Pages features quick links at the bottom for local restaurants, doctors, auto services and pizza, with a More tab which leads to other popular searches like plumbers, banks, grocery stores, etc. Unfortunately, you can't customize the popular search links, so if you find yourself frequently needing plumbers' phone numbers, you'll still have to go through the More tab.
Once you've found a listing you need, you can call the number directly, see a map of its location, or (I thought this was a cool feature) add the listing and all the info to your iPhone contacts, eliminating the need to look that listing up in the future.
Yellow Pages does have a few downsides. It has ads, which I personally don't like. They're relatively unobtrusive, but I'd like the option to pay for the app (it's free currently) and eliminate them entirely. Also, searches tend to be rather wide — a general search for restaurants in a given city will return literally a thousand results.
Yellow Pages also occasionally acted a little buggy with me. Errors when going through pages of search results, for example. That being said, over all I think it's the sort of application which will wind up being quite useful, and as I said, it's free, so there's really nothing to lose with Yellow Pages.