Remember Midomi? It was a first-wave iPhone app that identified songs, hummed or recorded, via microphone. It's since grown into a much more ambitious app called SoundHound - ambitious enough to actually replace your iPhone's music app. Now, it's free.
SoundHound's music recognition feature is still prominently featured, and it still does its job. Unlike Shazam, it will hazard a guess at just about any tune thrown at it, whether it human or speaker-created, and it'll succeed most of the time. Quick tests from my music library had both Shazam and SoundHound correctly identifying the track, though SoundHound, which evaluates whether or not to submit the track to its servers at a series of intervals, rather than after single, set period, was often faster. Shazam's catalogue did seem a bit deeper for more obscure tracks. But that's nothing new, really.
Where SoundHound really betters Midomi and Shazam is in music playback: You can access your entire library, except for podcasts, from the app itself. It effectively mirrors the iPod app on the iPhone and iPad, and adds all kinds of context to whatever you're listening to, from lyrics and artist info to YouTube video and links to the artist's Pandora stations. It's the iPod/iPad music player, except, well, more. And if you close it, playback continues through the default music player, meaning that even if you close SoundHound, the music doesn't stop.
A few minor quibbles: There is banner advertising, though you'll only encounter it in certain places in the app. I mentioned lack of direct podcast support, but SoundHound will play back podcasts if they're part of a playlist in your library. Lastly, and most conspicuously, is the song ID limit. You get five IDs per month, after which you either have to purchase five more IDs for $US1, or upgrade to SoundHound proper for $US5, which eliminates ads altogether.
But by all means, try the free version. I've actually switched to using SoundHound as the default music app on my iPhone and iPad, though I'll have to see if the lack of podcast support eventually forces a switch back. [SoundHound for iPhone/iPad]