Facebook 3.0 (iPhone)

Although Facebook is an application available on many different mobile platforms, none can hold a candle to the iPhone version, updated this summer to version 3.0. The latest mobile version of this must-have social networking application was so good, some even proclaimed that it was more useful and more usable than the Facebook website itself.

Designed by engineer Joe Hewitt (who later controversially declared he quit developing it and all other iPhone applications), Facebook 3.0 for the iPhone didn't just deliver a new way to socialize while on the go, but essentially became a portable "little black book", keeping you connected to your friends, events, and communication streams.

For those who don't spend their days behind a computer screen, the new app also made Facebook a more useful service, allowing you to quickly browse and upload photos or videos (the latter if you have the iPhone 3GS). Finally, the simplified layout, which displays just nine buttons in a grid-like pattern, is a testament to good user design, boiling down the complexity of Facebook to one easy-to-use interface that even the newest of mobile users can understand.

If any application deserves an "app of the year" award, it's Facebook 3.0 for iPhone.

Tweetie 2 (iPhone)

Another popular application for the iPhone was this year's revision to the Tweetie application. This "update" was actually a complete reworking of the app which introduced so many new features that the developer, Loren Brichter, decided to release it as an entirely separate application which costs the same as the original. This decision, in turn, led to a vicious backlash of complaints as Tweetie users whined that they now had to "pay twice" for the application. The claims for the most part were just ridiculous. The $2.99 price point was hardly a burden and no one was "paying twice" -- you were buying a brand-new application. However, the debate highlighted some of the issues Apple has with upgrade pricing -- that is, "no paid updates" are permitted. That left Brichter with no other choice than to recoup on his investment of time and energy that went into the building of Tweetie 2.0, but by charging again for the new version.

In the end, after the outcry died down, most everyone just forked over the piddling amount to get the new app, which introduced features like video tweets, offline mode, geolocation, and more. And nearly all are happier for doing so, too.

Twidroid (Android)

It's hard to not favor the iPhone in this list given the 100,000 apps now available for the platform, but Android apps deserve a mention too. Among the apps installed first by new owners of Android smartphones is Twidroid, the popular Android Twitter client. This mobile application has improved over time, and now offers a clean and easy-to-use interface where the most frequently used features (@Mentions, Direct Messages, etc.) are accessible via buttons available at the bottom of the app, no matter which screen you're viewing.

An updated version, which just launched today, now takes this application to a whole new level with its brand-new plugin architecture. Thanks to this feature, third-party developers can now extend Twidroid with their own services. This changes the application from being just another Twitter client representing one company's point of view as to what features it should offer, to being an app that's completely customizable and tailored precisely to an end user's needs. In fact, it was this last minute year-end update that forced us to add Twidroid to the list -- apps that support plugins may very well be the next big thing for mobile.

Foursquare (Cross-platform)

Last year, we thought the mobile social network to beat was Brightkite. While we still like that service, there's no doubt that Foursquare is this year's location-based breakout hit. At the beginning of 2009, the service was limited to only a handful of cities, but lately, that list has been expanding quickly to include a number of new cities worldwide.

Essentially, this geolocation-based service turns mobile social networking into a game. You "check in" as you arrive at new places in return for points, prizes, badges, and the honor of becoming the "mayor" of a place if you're the one with the most check-ins there. Additionally, Foursquare users can leave tips for others arriving to that locale so when they check in, they can see recommendation -- like the best entree at the restaurant, or where the nearest Starbucks is to that hotel, for example.

Some may claim that Foursquare's influence is still limited to the early adopter tech set for now and hasn't really become a mainstream hit just yet. Maybe that's true to a point, but considering the service just got a shout-out on The Simpsons not too long ago, we think Foursquare's days of being an undiscovered gem are limited.

Google Voice (Blackberry, Android)

Perhaps most notable for prompting an FCC investigation into Apple's secretive app approval process, Google Voice is one of the year's best mobile applications even if it's not available on the iPhone. According to Google, Apple rejected the app from the iTunes Store because it duplicated the iPhone's core functionality. Meanwhile, Apple claimed they were "still reviewing" the application because it alters the iPhone's functionality and user interface. The general consensus is that Apple isn't exactly being forthcoming here. A slew of other applications already available in the App Store "duplicate" the iPhone's functionality in some way, making Apple's rejection more suspicious.

As of now, Google Voice is still not available in the App Store. However, Blackberry and Android users are able to take advantage of this innovative mobile app which lets you set where your phone numbers should ring to, while also aggregating your voicemail from all your different lines. Those messages are then transcribed and emailed and/or SMS'd to you. Via the mobile application, your outgoing calls appear to be coming from your Google Voice number and not the number assigned to your handheld. This mobile app is so popular that it alone has caused some high-profile users to make the switch from the iPhone to Android.

Next page: Mobile apps 6-10, plus our readers' favorites!