Palringo is a free multi-client instant messaging app for mobiles that hit the App Store over the weekend, and it's the first to officially support Google Talk/Jabber, on top of Windows Live Messenger, Yahoo, ICQ, iChat and Gadu-Gadu (if you're in Poland). You can also use it to quickly send photos from your camera or photo albums to anyone on any of your buddy lists, who will see an inline image (if supported) and a link to a bigger (but still compressed photo) on Palringo's servers. Not MMS, but the feature works as advertised. It's great I can use GTalk on the iPhone now, but again, this is something we've been able to do via jailbreak and Installer.app for a while.
There were a handful of multi-client IM apps available via Installer, my personal favourite being Fring, which also has the ability to do VoIP calls over Wi-Fi. You can count on Apple putting the kibosh on Fring in the App Store unless it drops the VoIP feature due to SDK guidelines, which is a shame.
Palringo on its own sports a nice interface (which is a little jerky at times, though), and it tosses all of your new messages via any client into a universal inbox in the bottom-left corner. Like the official AIM client, it vibrates on message receipts, even if your phone is locked (and Palringo is still active, of course, which is another thing the Jailbreak apps had on it). On other platforms, Palringo supports voice chat over its supported protocols as well (not VoIP)—that feature's not in the iPhone version yet, but is forthcoming says the devs. [Palringo, App Review Marathon]