I have a confession to make: I don't use ringtones. Most of the time, my phone is on vibrate, and when it's not, I insist that my ringtones actually ring. However, if I were to use custom ringtones, Ringtonium is the app I'd use to set them up. This app is beautiful. In a way that few apps are. The interface is brilliantly easy to use and accessible to even the most tech illiterate users.
Very similarly to turntable, this app might remind some folks of iOS just a bit, although this is more due to a design philosophy than any particular UI element. The faux realistic textures are much more common on iOS than Android (and at least one texture is just plain lifted from the iOS folder background), however the editing interface more than makes up for it. Select a track, place an in and out point by dragging or using the snazzy scrub wheel and you're done. The app also allows you to record your own audio files.
Left: Ringdroid, Right, Center: Ringtonium. Past and future.
The resident king of the hill, Ringdroid, has been a mainstay of ringtones on Android for a while now. Unfortunately, the app hasn't been updated since March 2011 and it shows. Ringtonium brings a much-needed UI update to ringtone management.
The paid version offers additional effects you can apply to your ringtones. The paid version costs $2 and removes ads. So if you're even an audiophile when it comes to your ringtones, or if you really hate ads, it might be worth it, but otherwise the free version is ready for you via the widget below.