With our PJIRC opensource java applet, you can put up a page like this one, and its EASY.
All you have to do is cut and paste the follow code into your html. Then (described below) you can modify the settings to have your visitors join the channel(s) you want.
Note DO NOT download and host pjirc yourself! we host it for you, and it can't work if you host it without giving the user a nasty security warning. Just paste the code below, thats all you have to do.
Before you change any of the values above, try it out and make sure its working. Then, you can experiment with changes. The applet is configured by param tags which get passed into the applet.
Don't change the CODEBASE or HOST options, as they must be as they are defined to work properly.
Items commented out (<!-- ... -->) can be enabled by removing the comment.
If you want the applet to join multiple channels, seperate them with a comma (,) (but NOT spaces also!) for example value="#Debian,#Ubuntu,#slackware".
You can customize the various strings such as username, realname etc to your own needs, as well as the colors of the client (to limited extent).
Good luck, and happy chatting!