Chax: Pimp your iChat with this awesome plug-in
If you think that Apple's iChat is a bit limited, then you might find Chax interesting. Chax is a collection of minor modifications and additions that extends iChat and makes using it far more pleasurable. The preferences alone add several new features to iChat, then there are all the other benefits, which include a unified contact list, Growl notifications for new messages and status changes, and an activity log that displays contacts' status changes. You can also set iChat's contact list to be always on top and even send notifications to the dock icon.
The one thing that was really annoying in previous versions of Chax was the fact that for each different protocol - e.g. Jabber, Bonjour, AIM, etc. - you had to have separate chat windows. However, the most recent version has remedied this, meaning all contacts are displayed under one pane. It still doesn't make iChat feel as slick as Adium - especially the tabbed chatting - but Chax is a must-have for any iChat fan. For those that have always preferred to use Adium or another chat client instead of iChat, Chax may well persuade them to switch over.
