Apple’s App Store will celebrate its two-year anniversary this weekend, and the numbers its reached so far are staggering.

More than a quarter of a million applications developed and posted to the store. Five billion apps downloads. More than $1 billion in revenues paid to participating developers. But the next two years will be far more telling as the mobile app marketplace evolves. Apple, as usual, benefited from virtually creating the smartphone app store marketplace, and enjoyed first-mover status. But rivals are catching up, particularly Google’s Android system. According to a recent comScore report, Android is outperforming all other smartphone platforms in terms of growth.

While the overall smartphone market grew 8.1% in May over the previous three months, Android’s share of the market grew from 9% to 13%, while Apple’s share actually slipped a bit from 25.4% to 24.4% and BlackBerry fell from 42.1% to 41.7%. Microsoft, which recently killed its Kin smartphone line and won’t see its new smartphone software platform in phones until this fall, fell from 15.1% to 13.2%.

So what are the apps that are driving this growth, and what new apps and technologies will drive growth in the year ahead? Billboard is just one entity aiming to answer that question with our upcoming conference Mobile Entertainment Live: The Music App Summit, taking place Oct. 5 in San Francisco. A core part of that event is our first-ever Music App Awards, designed to seek out the best and most innovative smartphone apps created in the last year for these smartphones and more. Applications for the Music App Awards are being accepted through the end of the month.

Meanwhile, CNET lists a few things Apple, and frankly all app stores, could be doing better as the category evolves. These are important questions as the role of the mobile app versus that of the mobile web service is becoming an increasingly sharp distinction. YouTube, for instance, is pursuing a strategy that’s more focused on making its mobile website better for all phones can access. Its new mobile site has several features that the YouTube iPhone app doesn’t have, such as an easier-to-navigate library, better search, and the ability to bookmark videos.

Will web services like YouTube overtake apps? Unlikely. Both scenarios will play out as the mobile content environment continues to evolve. But it’s going to make for an interesting time to watch both competitors to Apple and competing formats battle to define the next two years ahead.

Mobile Entertainment Live: The Music App Summit takes place Oct. 5 in San Francisco’s Moscone West. It’s part of the CTIA Wireless Enterprise & Applications conference. Entries for the Music App Awards are being accepted now through the end of the month. Early-Bird registration ends July 16.