Shoutbox

Is there a plugin that'll let me add users from ICQ/Yahoo?  Kinda like Trillian or Miranda?

There is no plug-in capable of doing this.

You could, however, create a custom tab (see here) and point to ICQ2Go (ICQ's web client) or to Yahoo's Web Messenger.

you still can't send messages from MSN to other clients, but it gives you a chance to connect to them without opening another program. Keep in mind, though, that when you load another tab, your web client disconnects.