TimesToCome Mobile

You can download the update at HTC’s page but they now require you to sign up for an account.

Or from XDA Developers

I seriously do not know why Apple failed to include this. It wasn’t easy hunting down the various pieces but here it is Silence.m4r

Why would you want a silent ringtone? I use it for telemarketers, ex’s, and anyone not in my contact list. It simplifies life. Simple is good, Apple of all the tech companies should get that.

See also:
Make iPhone Ringtone free for OSX and painless to use.