Most of my chat contacts use either Yahoo! Messenger or MSN Messenger to chat. I have email with both, but no desire to have open two chat clients at the same time. Enter Trillian. It is not the tool of its kind, but I’ve been using it for a long time and have no complaints.
There is a free version and a paid version. I use the free and it is more than adequate for my needs. It has the flexibity to personalize, to log into one or all identities, to see emoticons as graphics, to sort contacts by friends or business associates (for example), to save history (especially useful when one receives instructions via chat).
If you use chat frequently, for business OR personal reasons, and your contacts are on multiple platforms, consider Trillian as your chat client.