Ever since the iPhone shook up the mobile entertainment space with the App Store, its competitors have rushed to create similar experiences for app developers and customers alike. While they remain far behind in the total number of apps created, particularly for music-based apps, all are starting to gain some real momentum.

Hot on the heels of Mobile Roadie’s expansion to the Android platform, now Slacker and Grooveshark have introduced some notable non-iPhone features today.

Customized Internet radio service Slacker updated the Android and BlackBerry versions of its app to include cached streaming capabilities. The feature allows users to store their radio stations on either device so they are still available even when the phone is not connected to a wireless network.

Grooveshark, meanwhile, is taking the path of least resistance and releasing a mobile app for the Palm Pre, which has the fewest number of apps available than any other platform save Windows Mobile. iPhone and Blackberry versions of the music streaming service’s app were released in January. A $3-per-month premium subscription is needed to use the mobile apps. Grooveshark settled a lawsuit with EMI last October but was sued earlier this month by Universal Music Group for allegedly using some of UMG’s pre-1972 catalog without permission.

Devices like the BlackBerry, Palm Pre and Android phones will need an abundance of apps to catch up to the iPhone’s lead. According to Dutch analytics firm Distimo, the iPhone leads the pack with more than 150,000 available apps. Android devices are a distant second with just over 19,000. Nokia’s Ovi platform is third with more than 6,000 and BlackBerry just behind it with close to 5,000. Palm and Windows Mobile have 1,492 and 693 respectively.

However Android is catching up fast. While Apple adds a over 13,000 new apps a month Android is adding around 3,000 a month and growing, making it the fastest-growing store of all.

For a more detail on the Distimo’s app store breakdown and analysis, presented at last week’s Mobile World Congress, visit the company’s blog.