We use Google Talk a lot at TCG.  It's not the most feature laden instant messenger out there, but it's simple, easy to use and compatible very simply with my BlackBerry, allowing us to stay in contact where ever I'm at.  Does Yahoo and Microsoft have messengers that can be loaded on your BlackBerry, yes... but Google Talk is just nice and simple and it works great for us.

If you already have a Gmail address, you already have the capability to use google talk, but you might not know it.  Gtalk loads in the left column as part of your contact list.  If any of your contacts have gmail and they happen to be logged in to their gmail account at the same time, you can click their contact name and chat in real time with them!

That's great, but we actually don't use gmail a lot, therefore we're not logged into it very often.  That's where Google talk comes in to play.

You can simply go to http://www.google.com/talk and download the instant messenger right from there.  Install it with a few clicks, log in with your gmail details and you're in!  Now you can invite others and add others' Gtalk usernames just like any other instant messenger.

It works great for us... simple, quick, light, mobile... maybe it will work for you.