From ModMyiFone - Wiki
Originally listed on TUAW by Erica Sadun, these are the mobile protocol handlers on the iPhone. Opening a URL preceded by these protocols does the following actions:
itms:// Launches Mobile iTMS store.
maps:// Launches Google Maps. (iPhone only)
feed://, feeds:// Launches dotMac reader
tel:// Dials a number, specified in the URL. (iPhone only)
mailto:// Launches the iPhone mail app. (iPhone only)
youtube:// Launches YouTube.
Other items that were listed but do not seem to work are: calshow://, callto://, vmshow://, prefs://, nowplaying://