There are a lot of Instant Messenger on the Internet. Most of them are desktop based, which you need to install on your system. Not to mention that your chat buddy must have the same IM application installed on their system. If you need more practical chat messenger, there's a cool stuff for you. It's toksta.
Toksta is web based instant messenger. You don't have to install any desktop application. Just put the code on your website. For wordpress user, there are also ready made plugins available for Wordpress. And if you familiar with PHP, Ruby, or JSP. You can embed it using those programming scripts.
Should be great for a blog, forum, or social network website ;).