With 150,000 software downloads now available for smartphones, the most useful app you can own is one that lets you discover ... other apps. Of all the handset manufacturers, Apple puts the most effort into helping users in their quest for new software. Searching through the top free and paid-for items in the App Store gives a good snapshot of the most popular downloads of the moment. The Genius function, which makes recommendations based on what you already have installed, works pretty well with music - you obviously like jazz, so here is some more - but is chaotic when it comes to apps. I own the Nasa app, therefore I would also enjoy reading about the lives of US presidents? Actually, no.
More useful is AppsFire (free). After installing software on your Mac or PC, you can recommend any of your apps to an email contact or Twitter follower from that computer or via an app for the iPhone and iPod Touch. Also sharing their favourite apps in the "VIP area" are a bunch of tech gurus and bloggers. Stephen Fry has yet to turn up, but it's only a matter of time.
AppReview (free for iPhone and Touch) claims it is put together by independent reviewers, although many of its recommended apps seem to coincide with the prevalent ads on display. Prices are all in dollars and categories are very US-centric. Avoid.
iPhone and Touch gamers could add iPhone Arcade to their Facebook page. This free application shows trailers of games and lets you recommend them to friends over email, Twitter and, of course, Facebook. Don't bother with it for any other categories of app - it simply parrots their App Store sales pitches with prices in dollars, making it less useful than simply searching through iTunes.
For once, app-hunters with Android mobiles are being better served than the iPhone-toting hordes. Sidebar (free) builds up a snapshot of a user's interests by asking questions, then sends up to a dozen relevant recommendations every day, including video, music, games and ringtones. It's not perfect - again, there's a US bias and some ads to wade through, and it uses annoying pop-ups - but it will be available for Apple, BlackBerry and Windows Mobile devices soon. Until that day, Android users get to enjoy a rare sense of superiority.