Facebook includes an awesome chat feature that lets you instantly start conversations with your Facebook buddies without installing any IM software but the only small issue is that you need to be on the Facebook.com website in order to use Facebook chat.
If you find this a bit inconvenient, here are some good workarounds that will let you chat on Facebook from anywhere without even having to open the Facebook site in your browser.
Facebook Chat Inside the Web Browser
If you have Google Chrome or Prism for Firefox, you can open
this address in your browser address bar and then create an application shortcut to Facebook chat on your desktop.
Firefox users may bookmark the above link and set the "Load this bookmark in Sidebar" property to true in order to use
Facebook chat from the Firefox sidebar.
If you an IE user or hate to use Facebook chat from the sidebar, try
Gabtastik – this is a minimal desktop client for Facebook that looks exactly like the web version of Facebook chat but will also send you notifications of new messages and chat requests from the system tray.
Meebo, the very awesome web based instant messaging service, now supports Facebook chat as well. You can connect your Facebook account with meebo via the Facebook Connect service (so there’s no need to share your Facebook credentials with anyone) and instantly chat with your online buddies inside the web browser.
There are two advantages here –
you can use Facebook chat even if the main Facebook.com site is blocked in your office and two, you can simultaneously connect with friends on other IM networks from the same website.
Facebook Chat Clients for your Desktop