I spent a while researching this recently, and after having tried out four different packages, I chose X7 Chat:
http://www.x7chat.com/
Doesn't have any ads, however it does require MySQL - fine so long as you have. Also I've noticed there can be up to 4 seconds of lag between you hitting enter and the other chatters getting the message - and it wasn't a bandwidth issue since I use it on a LAN. All in all though I think it's the best compromise.
Another one I used, quite a while ago before finding X7 Chat was Blablite. I can't easily find it on Google but I'm sure it's there; I just get example pages:
http://www.google.co.uk/search?hl=en&q=blablite&btnG=Google+Search&meta= If you like them you could investigate further. Blab may well be more what your looking for, since it doesn't need MySQL (iirc) and is generally much more basic. I can't remeber how loading times and bandwidth compare however because it was a long time ago.
Hope this helps