I'd forgotten about that era of my life! I used Adium on OS X and had customized so much about it, down to the individual sounds played when certain friends (and crushes) came online. And I was able to essentially ignore what platform I was using to talk on: AIM friends and GChat friends appeared side-by-side. What a dream.

Trillian here for me! That shit was AWESOME. So much customization too of the UI.

I didnt realize I had trillian setup to log all my conversations to file and I found the logs recently when going through a very, very old harddrive. Middleschool me was cringey as fuck

Loved Adium.

Upsides: I no longer had to care what messenger my friends used, it was all the same to me. Extensible and theme-able which was just the way I liked my software at the time. Growl integration.

Downsides: I made the mistake of installing and using a FFVII-themed sound pack that invoked a Pavlovian response in me every time I heard certain sound effects from that game. Took the better part of a decade after I stopped using Adium to shake that off.