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The Contact List

Lets have a look at a well laid out contact list.  The dimensions of the main window should allow the user to have it remain open beside other applications and always remain visible.  Since the data is a list by definition, it is naturally convenient to have a tall skinny window, but there is nothing to stop you have the window display two or more columns if the user drags the window wider. 

Information like user status, your own status, and waiting events should be available by one quick glance at this window.

Groups

Jabber wisely organises all users into groups.  Each group should be collapsible so that you can focus on only the people you are interested at the time and conserve screen space.  For example, if you are at work, you will probably have your "Friends group" collapsed, as you are unlikely to chat with them while you are working.

The right arrow for a collapsed group and the down arrow for an expanded group is a good convention to follow.  If an event comes though from a contact in a collapsed group, the 45° arrow should be shown to indicate that one or more users are temporally displayed even though the group is collapsed.

The sorting of contacts by status is very important for a Jabber client.  Most clients I have seen simply have the contacts sorted alphabetically within each group, with an option to show or hide offline users.  This might be fine for simpler systems such as MSN and AIM, but since it is possible to send messages to offline users with Jabber, most of the time you will want to have access to users who are not currently online.  My strong preference is to have two versions of each group, one for people who are online, and one for people who are not, as per the illustration.  This allows the people who you want to contact most often (people online) to be sorted near the top, so they are easy to access, and you can still scroll down to get to people who are offline.  It is also good practice to include an option to move the name of the contact to the top their group when a message is received in order to make them easier to locate.

Online and offline contacts should be differentiated by colour and/or icon.

Title Bar

Instant Messaging clients are typically long and skinny, so usually the space on the title bar is fairly limited, but can still be useful.  The space in the title bar is probably too short to effectively display the JID that is currently logged on, but this is very useful information especially if multiple users are sharing the same machine.  The close window widget (X) should not actually exit the app, but should minimise the contact list window just leaving the system tray icon.  The user will consider its function to close the actual window, not exit the actual program and will avoid novice users exiting the application accidentally.  Truly exiting the app can be done through the popup menus, or by right clicking on the system tray icon.

I would recommend a simple way to change a window from "always-on-top" or to send the contact list to the back, to enable the user to easily get to part of the workspace that may be obscured by the client.