OctroTalk is an application for Windows Mobile smartphones and Pocket PCs that lets you chat and talk to your Google Talk contacts. The software uses Jabber transports to let you connect to other IM networks (Yahoo, MSN/Windows Live, AIM), but you can only chat with contacts from these networks. OctroTalk uses tabs so you can have multiple conversation at the same time, supports peer-to-peer file transfer and folder sharing.
There's also a desktop version of the program that allows the Smartphone client to transfer files remotely from your computer.
The software is free during its beta state and it's unclear if you'll have to pay to continue to use it when it comes out if beta. Oh, and you need to have Windows Mobile 5+.
There are many other ways to use Google Talk (including Java applications that work on most mobile phones), but Google should really provide a mobile Google Talk app or at least a non-Flash version of the gadget. And an update to the Windows client. And ports for Linux and Mac OS X.