Want to host your own Instant Messenger? Then you should try out Ajax im. This is very compact custom instant messenger which is based on PHP and AJAX. The complete download is less than 300kb! In order to host this on your server, you need a MySQL database and PHP support.

Since this is AJAX based, you can integrate this within your existing site. You can also host this as part of your intranet. It is not as sophisticated as any of the commercial ones, but still is good enough as supporting tool to your site.  You can try out a sample implementation here.