People have been recommending music on twitter for a long time with hashtags like #nowplaying and through apps that let you share your recommendations on twitter like Soundtracking or NowPlaying. The problem with most song recommendations on twitter is the lack of a preview. Recollect.fm makes it insanely easy to recommend a song and also does it beautifully.
Recollect.fm is tightly integrated with twitter and while some apps let you post a link to a song you recommend, this one goes beyond that and creates a stunning web page with large cover art and a song preview (if available) for your recommendation. The universal iOS app is really good. You can search for the song you want to recommend, recommend the current song that is playing on your device or search through your media library on the device. The app lets you choose the album cover from an image search that specifies the quality. Adding information like album name or artist name to the tweet recommending the song is easily done by tapping the respective buttons above the keyboard. You can browse through recommendations in list form or as a vinyl bin with in app song previews. Minor details like the gloss effect on album art work is what sets this app apart from the others. There is also a link to buy each song recommended on iTunes. Just like any other media sharing app, you can look at what is popular and see what your friends on twitter using the app have shared. You can collect other recommendations you like which basically saves the links to your profile.
My only real aesthetic issues with Recollect.fm are in the iPad application. The detail popups for each recommendation look too stock iOS compared to the rest of the app. The lack of Retina iPad support is disappointing. Also it is absolutely a crime to make your user log in to twitter the old fashioned way when iOS 5 has such easy twitter integration for apps. I would also hope they let you filter your stream by genre in a future update. Barring these issues, Recollect.fm is a great concept executed near perfectly. The universal app is available for free on the App Store.