New app called Appapella, it allows people to sing and rap over instrumentals you can then add a audio filter, similar to how you would add a photo filter on Instagram, then instantly share the recording via Twitter, Soundcloud, SMS, Email or Facebook.

Here is a example of a user using it and instantly tweeting their produced track on twitter.
https://twitter.com/smokerule/status/479975634848862208

Here is a example from a user with the filter applied.
https://twitter.com/HelloAudibase/status/481035198008922112

List of filters:
Low Pitch
High Pitch
Add Reverb
High Pass Filter
Echo Effect
Chorus Effect
Boast Vocals
Silly Low Pitch
Silly High Pitch

It's a great way for people to either have fun or create versions and collaborate and share with the artists who have submitted the instrumentals.

Features:
Collaborate With Artists
Layer your vocals over one of our great instrumental tracks provided by talented musicians.

Add Audio Filters
Choose one of our great audio filters to layer over your vocals and enhance your track.

Save and Store Lyrics
Save your favourite lyrics so they’re easy to find for future recordings.

Share Instantly
Easily share your produced track with your friends via Twitter, Facebook, email, and text.

Attach Lyrics to Tracks
Attach the chosen lyrics to the track you’re recording for a mistake free result.

Add Your Own
Get yourself involved! Submit your instrumental tracks or lyrics for use on the app.