With the release of the iPhone 2.0 software on Friday and the opening of the App Store -- where hundreds of third-party applications are now available -- Apple's iPhone has taken a major leap toward becoming a premiere mobile platform.

Since the iPhone's arrival last year, one of the constant complaints about it was that the hardware was a technological marvel but the software was lacking. A little over a year after its debut, Apple released not only the most significant hardware upgrade since the iPhone's inception, but also the most significant software release to date. Best of all, the apps work on the first-generation iPhone as well as the newer iPhone 3G, and on the iPod Touch. (You have to pay US$9.95 to update the iPod Touch so it can run third-party applications, though.)

While new hardware is important for attracting new buyers -- what with a lower cost of entry and the 3G chipset -- it's the software and the arrival of the App Store that allows the iPhone to transcend being just another mobile phone. Officially opened on July 11, though accessible the day before, the App Store sprung to life with more than 550 third-party applications ranging from games to sketch pads to medical applications.

To get you started, we've chosen 11 applications that are decidedly worth looking at, though you should check out the App Store yourself and weigh in below with your own favorites. The first six are free; the rest will cost you a few bucks. All are worth a look:

AOL Instant Messenger (free)

This handy application provides a fast and easy way to talk to your peers via AOL's instant messaging system, as well as buddies on ICQ and .Mac/MobileMe. Already one of the most downloaded applications on the App Store, this program is a simple and straight-forward way to connect to the AIM network with your iPhone.

Upon first launch, the application presents you with three login options: AIM, MobileMe, and .Mac. Once the service is chosen, username and password is entered, you're presented with your Buddy List groups, which, upon selection, displays your buddies, their availability and their status messages. From the main Buddy List, there are arrows beside your Buddy names that can be tapped, bringing you to a Buddy Info screen. From this screen, you can change their display name, the group they're associated with, and even add your buddy to a Favorites list for even easier access.

This application uses the swipe gesture to switch between active chats, and the entire app is straight-forward, taking full advantage of the available screen real estate without clutter. This app is easily on my own personal must-have list, especially in light of AT&T's SMS price-gouging on its new monthly plans. (Just because AT&T isn't the only carrier to overprice on SMS doesn't make it any less wrong.)

There are a few caveats, though. AIM doesn't tie in with your Address Book and it does not run in the background. Once it's quit, you're logged off.

Apple is already working on that last issue and is devising a notification API that will allow messages and alerts to be pushed from their servers to a person's iPhone. That will eventually allow IMs to show up as alerts, just like SMS messages. Another feature this API will bring is the ability to display badges on Home Screen icons -- just like Mail does. Once released, this API will provide immediate IM notifications without cutting into battery life or wasting processor cycles, both of which are inherent to background applications. It also showcases Apple's commitment to making the iPhone a better mobile platform.