Built-in Map app
The built-in maps application kind of has this feature. Load it up, tap the pulsing blue dot when it gets a fix. A banner should pop up saying "Current Location". Tap the arrow at the end of the banner, and on the new screen there is a button to "Share Location". You can send your location by MMS or email. It will be sent as a vCard file with a URL to the location in Google Maps, which on the iPhone will load up in the Maps app.
Of course, if you're in a mall or something you might not be able to get an accurate GPS fix.
If you want to specify a location instead, you can drop a pin and then follow the directions above with the pin.