Searching the App Store using iTunes doesn't work very well, and there are plenty of sites that review apps (including TUAW), but there are so many apps out there that finding what you want can be a matter of luck rather than skill.
That problem has largely been solved with Appsaurus [iTunes link], a U.S. $0.99 app that can quickly focus on what kinds of apps you like and make decent suggestions for new apps to try or buy.
Here's how Appsaurus works. When you launch the app for the first time, it presents you with an explore mode. It displays a list of apps, and you tap the ones that are of interest to you. You may have some of the apps, but you probably won't have all of them. When you select a favorite you'll get a new list that includes apps that match the general description of the app you clicked on. Run through a few iterations of this process, and the app begins to learn your preferences.
If there isn't anything on the list that appeals to you, just ask for a new list. If the title of the app doesn't give you enough information, tap on the right side of the title, and you're transferred to the app store. You can actually buy the app if you want to. When you tap on Appsaurus again you'll be right back where you left off. As you explore apps you can mark them as favorites, or tell Appsaurus you don't want to see an app again.
In no time at all I had some great picks, and bought a few apps that I simply would not have discovered any other way. However, that's not the end of the Appsaurus playbook.
You can create custom searches, like "all free chess games," or "photography apps under $0.99." You can save those searches, and then let Appsaurus do the heavy lifting and find them.
In my experience, the app worked very well, was easy to use, and was well executed with an attractive GUI. If there is a downside, be prepared to spend some money on apps you find; I was glad to find them, though, and don't think I would have encountered them without the guidance from Appsaurus.
Of course the app store on the iPhone has a 'genius' function built in. It scans the apps you have and makes some recommendations, but they get a bit strange. It suggested I get an app for a Chicago TV station because I had CNN. Well, they are both news apps, but interest in Chicago news doesn't necessarily follow from my interest in general news.
Appsaurus isn't a big investment. I'd urge you to give it a try and see if it gives you some intelligent navigation through the rough seas of app finding.
Click below for some screen shots of Appsaurus in action.