Leafsnap: Leafsnap is the smartphone user's secret weapon to a more meaningful relationship with trees. It's incredibly simple and strangely addictive: find a tree, grab a leaf, snap a picture. The app scans its outline-parsing criteria like smooth or jagged edges, single- or many-lobed-and tells you what tree it thinks it came from. It's similar to the "whoa! this is the future" feeling you get when you deposit a check via smartphone.
Hound: Hound is a voice recognition app that's essentially a really, really good music-specific search engine. You speak an artist's name or song title into the app and it'll pop out all the stuff you want to know. The voice recognition part of Hound is great-it's the future!-but the results page of Hound is what will keep it on your phone. Like SoundHound, Hound will spit out a ton of information for you. If you're searching for a song you'll get lyrics, YouTube clips, an iTunes link, a Pandora station link, ringtones, similar artists and sharing options. If you're looking for an artist you'll be loaded with their bio, concert dates, a full discography and more. Free
Vocal Zap: Vocal Zap is a music app that strips (or lowers) the vocal track from songs in real-time. It keeps the music intact so you can sing karaoke any time the mood hits you. The software does all the processing on the fly and lets you toggle the vocals on when you need a little help to hit a difficult note and off again when you want to fly solo. Free
Foodish: Foodish is attractively simple, there's your pictures of what you ate casually displayed as if it was on a board, an option to add more photos and a chart to see your happiness (or health meter). You can swipe up and down the screen to see what you've eaten for the day, swipe across the screen to move from day to day, or fast forward a week with onscreen buttons. Think of it as a food photo journal. $2.
Sonar: Sonar is designed to tell you about the connections that exist between yourself and the the other people nearby. It connects with your Facebook, Twitter, and Foursquare accounts, and tells you what kind of social media connections you share with other people also using the app nearby.
eBay Motors: eBay Motors has its own iPhone app and not only does it let you search and shop for cars, check vehicle history reports, scan your VINs to create a virtual garage, and save searches for your dream car, you also get to be that guy who said they bought their car from a freaking phone. Get the app, some cash, and it's all you. Free
Audibly: Audibly is an app that offers 2,947 classic audiobooks catalog for free. The app uses narrators from Librivox, and from a few quick listens, they all sound perfectly fine. Of course you won't be able to listen to the most current of books (so no Twilight, sadface) but it's still a nice selection of books that're free in the public domain: Shakespeare, Benjamin Franklin and Teddy Roosevelt autobiographies, Sherlock Holmes, Sci-fi novels and a ton more.
iTransmission: iTransmission is a torrent downloaded that lets you download torrents straight to your iPhone. It's jailbreak-only, which means you'll have to head over to the InsanelyI Cydia repo (repo.insanelyi.com) to get it. The app is currently in alpha, so it might be a little buggy, but it's free and fully featured.