One communication protocol that is not supported out of the box on the iPhone is IM/chat. While the inability to run background applications on the iPhone limits the true effectiveness of IM, BeejiveIM definitely does makes the most out of what is available.
BeejiveIM is the most reliable and user-friendly mobile IM application on the market for your iPhone or iPod Touch, featuring desktop-like instant messaging and always-on efficient networking.
Works with AIM/iChat/MobileMe, MSN/Windows Live, Yahoo!, MySpace, GoogleTalk, ICQ, &; Jabber.
By far the biggest nock against BeejiveIM is the price. At $15.991 it is the most expensive application on my iPhone, but in this area there really are not a lot of alternatives (I tried Paringo, but it never worked for me.) So far it has been very solid for me, so if you are OK with the price, I definitely recommend checking it out.
1 I did get it for a couple dollars less when it was on sale.