Talk with people you know, all around the world with iChat.

iChat supports several protocols like AIM, ICQ amongst others. Using a Jabber transport, users can also integrate their MSN, Yahoo! and Google Talk contacts into the Jabber pane. The latest version of this application (10.5 as of this writing) use Apple's Aqua interface, speech bubbles and pictures that are meant to personify the online chatting experience.

Although this is a competent application I prefer using AdiumX. Best of all, it is free, open-source, slick and tasty. In the end, it is your choice. You could try out both applications to see what one you like. It is free so you do not have to pay anything, anyway.

iChat For Windows?

I am afraid not ... but you can download Miranda. That is a good and free IM application for Windows users supporting a lot of IM protocols.