When the iPhone came along, one of its key selling points was that it was an iPod, but with phone capabilities. The iPod was very much at the forefront of its design.But over time, as the smartphone revolution has kicked in and the web, and particularly the cloud, have become more important the iPod app has taken a back seat with a raft of innovative web-based apps taking over.And, there's one app that could even spell the end of the iPod app. Yep, today's App of The Day feature is centred around a real iPod app killer.....Spotify\nFormat\niPhone \/ iPod touch\nPrice\nfree (but you'll need a Premium subscription)\nWhere\niTunes\nRecently, Spotify announced a 7-day free trial period for its Premium service (which is priced at \u00a39.99 per month) so we thought to ourselves, what better time to feature the brilliant Spotify for iPhone app?After all you'll have free access as a Premium user throughout the duration of your trial (and beyond, should you choose to take the premium-plunge).\u00a0If you're a Spotify user already, then you'll have no problems using the app as it's as straight forward as the desktop version.\u00a0\u00a0Your account is synchronised across all your Spotify platforms (with sync-on-the-fly) so if you create playlists on your PC, they are copied over to your iPhone and vice-versa. Also, all of your account details are automatically copied as well.\u00a0As well as your playlsits, you can search for artists, albums and tracks as per the desktop version, and play tunes straight away, with streaming over Wi-Fi and 3G possible. The Wi-Fi streaming is obviously a lot more reliable than streaming via your phone network, but we found that 3G streaming (on the Three network) was more or less spot on, with just the odd stutter.\u00a0But, if you don't fancy taking your mobile phone's data plan to town, you can always choose to go offline, where you'll have access to any playlists that you select to be available in this mode. And we're not talking about a handful of songs here, we're talking hundreds of albums and thousands of songs. 3333 songs to be precise, so about 300 albums.\u00a0\u00a0This really is a killer feature of the app, as it gives you an incredible offline catalogue to choose from. You can choose to sync the playlists (i.e. actually download them to your device) via 3G, although you can turn this option off, but it's best to sync over Wi-Fi at home before going out and about.\u00a0\u00a0If you choose to sync at high-quality then you're basically building a stored music collection (that will last as long as your subscription does) to rival your iPod\/iTunes one. You can even choose to sync whole albums to offline, thus keeping your collection nice and organised.\u00a0\u00a0Be aware though that you can't organise the order of your playlists via the app, this all needs to be done via your Spotify desktop client. You can create and amend playlists from within the app though.\u00a0As well as catering for all your streaming needs, and providing a mountain of offline music, the app also jumps on board with Spotify's social network features - with Facebook, Twitter and Last.fm features all built in. You can also keep up to date with all that's going on in the world of Spotify via the What's New section, which links to the Spotify Twitter stream.\u00a0If you're a Spotify Premium user there's absolutely no reason not to have the Spotify App on your handset (apart from the iOS version featured in this post, there are also versions for Android, webOS, Symbian and Windows Phone 7).It puts an enormous wealth of music at your fingertips, ad free, and with the large offline quota you needn't worry about hammering your data plan, or losing your signal.\u00a0Quite simply, it's a brilliant app, and if you're not currently a Spotify Premium user, then the prospect of Spotify on your mobile could well be the incentive that you need.And don't be put off by the large amount of 1-star ratings on iTunes. The majority of these are people that downloaded the free app, but were unable to play anything without a Premium Spotify account.