I discovered one called PJIRC (which is listed on one of the sites c0ldfusi0n pasted) yesterday, and I think it is very nice. It has some features that other java chats lack like the ability to chat in multiple channels at once. It has a "sorta" mIRC feel to it, different, but still nice. The best part, it's free. The worst part, it's not signed which means you have two choices, you can either sign it by buying a certificate (costs about $200-$300), or you have to run the java applet on the same machine as the IRC server. To connect to a network like DALnet, that makes this impossible unfortunately.
Another one (that I didn't see on the sites posted), is CGI:IRC. It does not use Java so there is no applet to worry about, it works fully on Perl (and some Javascript). The downside is, every user that connects using the CGI:IRC interface will have the same hostname, this makes it very hard to ban people. But if you are interested,
http://cgiirc.sf.net 
