Price: free Download on the App Store
Ringtone Recorder - While the iPhone offers numerous features and customization options, most ringtones (officially-sanctioned ones, at any rate) have been limited to songs purchased in the iTunes store which are enabled to be used as ringtones. Of course, there's certainly a wide selection, so it's not like iPhone users are hurting for ringtones.
However, in case the iTunes store just isn't cutting it for you, Ringtone Recorder by Fun Mobility allows you to personalize your iPhone a little further by converting clips you record from your phone into ringtone-ready files. Have your significant other record a sweet message if you want, or maybe you'd get some strange pleasure out of a ringer of your boss yelling at you? (Masochists need ringtones too, folks!)
Ringtone Recorder is extremely easy to use. To record, simply open the app and hit the giant red button that says — you guessed it! — "REC." Record whatever it is you want, and hit "STOP" once you've finished. After that, hit Next, which allows you to preview the file and, once you're satisfied with the recording, to name it. After you've named your file you can then choose "Download It" to have it converted and sent to your email. (From email, you simply download the file, import it into your iTunes library, and add it to your iPhone as you would any normal ringer.)
There's also the option to share ringtones by sending them to a friend's phone number or email address, or to simply save them to your phone for later sending/use.
When I tested Ringtone Recorder, it worked very well for me. The audio I recorded was clear and the email with the ringtone I created arrived within seconds, and the file was perfect. And not that I would ever suggest doing this, of course, but a clip I recorded of some audio that was playing on my computer speakers was pretty decent, although a bit tinny.
All in all, Ringtone Recorder is worth a look. It's probably not indispensable, but it has the potential to be fun and maybe come in handy.