I use Adium on the Mac and Trillian on Windows. Both excellent applications. I am not able to use gTalk on Trillian but that's fine.
How about the iPhone? Well, there are plenty of IM options on smartphones but until recently, the iPhone was mostly limited to web-based chats. More recently, Google graced us with gTalk for the iPhone. But once you closed Safari, you will not be able to able to follow the conversation.
Now that iPhone 2.0 is out, that IM options, dedicated IM apps are now available and more will likely follow. One is Palringo. It's been around on other platforms for a while and it has its own services it's trying to push. Palringo's iPhone app allows the user to sign up with Yahoo, MSN, gTalk, ICQ, and other services very much the same way Adium and Trillian does.
There's a catch. Palringo's own services offer pictures and voice messages so you need to sign in with a Palringo account first. That bugs me a bit. But the service is free. I've been using it for a couple of days and it's quite robust.
However, users of MobileChat can't agree with the robustness of the service. MobileChat 3.0 release just yesterday (August 12). While it has managed a three star rating, server problem has kept it from even a higher satisfaction among users. Plus, $3 is a bit much for consider that its competitors AIM and Palringo are free.
MobileChat has a feature that is both a plus and a minus at the same time. MobileChat's servers would keep your accounts logged in even when you're not. Wonderful. But you need to trust that they do everything to secure their servers. This is likely the case with Palringo as well.
Oh, I almost forgot about AIM app. It works well but likely most people, we have multiple accounts across different services so AIM doesn't work for me.
With these two apps and web app gTalk, we're not there yet. The limitations quickly become apparent. The UI can be improved more. I like to see the ability to swipe with my finger between conversations like I can with the menu. I also like to have have a small window pop up to let me k now there's a new message and an option to go directly to that conversation.
There is another issue with staying connected but that should be solved once Apple releases and up allowing background message alerts to apps.
I don't see myself using IM apps that much on the iPhone. Perhaps, it is because I never really text on my phone and there's no one I really really have to talk to all the time. But it's good to have the function handy when I do need it.