One of the non-obvious parts of Adium's interface from the screenshots was that it integrated with Mac OS's calendar app, so I could have a contact "John Smith"

One of the non-obvious parts of Adium's interface from the screenshots was that it integrated with Mac OS's calendar app, so I could have a contact "John Smith" and enter his AIM name, his Yahoo name, etc and they'd all show up as "John Smith" and be collapsed into one entry in your contact list. On top of that, Adium is/was also incredibly themable. You could use independent Adium Xtra addons for both your contact list and your chats. I had a theme so that all messages just appeared sequentially like so: [Hello][Hello][What's Up?][Etc.] where mine were red and the person I was talking to was in blue (or vice versa, it's been awhile). You could see so many messages at once. Adium is the application I miss most from MacOS, although I imagine I'd still miss it even if I were still using a Mac. With the exception of the addition of multi-device continuous chat history, we've really regressed in the category of instant messaging since the days of multi-chat clients.