This used to be one of the biggest Mac open source projects. I remember they demoed project builder (before Xcode) compiling it
I had amsn when msn and yahoo messenger were the place everybody I knew was at.
I was on Miranda. It was tiny and portable (unlike Trillian) and could connect to pretty much every popular messaging service, including IRC. And in spite of being able to connect to everything, it used far lesser memory than even just a single instance of MSN and Yahoo. It was one of my go-to apps that I'd install on a freshly formatted PC back in the day.
Loved Adium and the community was great in building custom smileys (For the young people, that's how we called Emojis back in the days!), themes and dock icons.
One of my happiest open source memories was creating an Adiumy skin for my college and others actually downloading and using it. The app was huge. Those were the days.
Best chat client ever. Man we had it good back then.