If you have trillian, you can use that. My server does not have a web chatroom, so as far as I know you use a program.
Some use mIRC however it is only free for 30 days then you have to pay $20 for it. If this is ok with you, it is supposed to be a very good program. There are several other programs you can use, however I'll tell you don't even bother with IceChat because it does not appear to be able to connect to my server. Visual IRC is supposed to be good, however it froze my computer a bunch of times. I'm sure someone else can recommend a good one, I just use Trillian it is free and effective (it also does AIM MSN YIM and ICQ though ICQ never stays connected for me.)