Gajim (Wikipedia article) is a free/libre XMPP client. It exists since 2004 and is still maintained.
It’s available for BSD, GNU/Linux, and Windows. I’m using it as my primary XMPP client on GNU/Linux. I tested it on a Windows system once and everything seemed to work fine there, too.
It doesn’t seem to support XEP-0084, but it supports avatars based on XEP-0153: vCard-Based Avatars.
You can find some screenshots on the official website, e.g.:
There is also a mode where the roster window and the chat window are combined into one window. I made a screenshot of my client (sorry, it’s in German and taken on GNU/Linux):
(On the left is the list of my accounts. Each account entry can be expanded to list its groups/contacts. On the bottom I can change the status. If chatting with multiple contacts, each chat is displayed as a tab.)
It comes with a few alternative themes and icon sets.