From IPhone Dev FAQ

Contents 1 URL Protocol Handlers 1.1 Built-in URL Protocol Handlers 1.2 Public Third-Party Protocol Handlers 1.3 Links

URL Protocol Handlers

The iPhone has full support for protocol handlers (just like OS X). Third party apps can register custom protocols handlers and have their app open given a specific url in Safari or in another app. This can be used for a simple form of inter-process communication on a single iPhone. (See: Remote Procedure Calls for an article on communication by apps on different devices.)

Built-in URL Protocol Handlers

 * http/s (MobileSafari)
 * mailto (Mail)
 * tel (Telephone)
 * youtube (YouTube)
 * itms (iTunes Store)
 * maps (Maps)

Public Third-Party Protocol Handlers

Protocols registered by apps that are intended for use by other apps and the general public.

 * twitterific (Twitterific)  - someone want to say how its used?
 * twit (Twittelator, Twittelator Pro)

Links

 * Choose your Own iPhone URL Scheme
 * Inter-process Communication
 * 2 Way App Integration