Google Voice
Well, it's about a month and a half behind its Android counterpart (and what feels like decades behind the web interface), but the iOS Google Voice app finally (finally!) has support for multi-recipient texting. Version 1.4.0.2372 adds few other nice tweaks, including one touch copy and paste on the dialpad and a larger text entry field with a character count but, obviously, the mass texting capabilities here steal the show. So hit up the App Store to update now and start spamming all your friends.