Windows/Mac/Linux (Firefox): Web-based instant messaging service Meebo connects to any and every IM chat client you can shake an avatar at, and now the popular in-browser chat application integrates tightly with your Firefox sidebar with the new Meebo Firefox extension. The extension supports visual IM notifications and drag and drop link and image sharing directly from web sites in addition to other already existing Meebo features—including file sharing. There really aren't any robust, fully cross-platform chat applications out there (though there are a few great platform-specific apps), but with this new Firefox extension, Meebo is getting close.
After you install, you can either sign in to accounts individually or sign in to your Meebo account, which holds all of your different account credentials so you can sign in to multiple IM accounts at once.
Once connected, you've got a simple contact management pane in the sidebar that you can view or collapse easily. You can even drag web content onto contacts names to automatically open a new IM and share the link or image. In the meantime, your IM conversations all take place inside a Meebo tab, which is automatically opened when you sign in. The Meebo extension is free, should work wherever Firefox does. I know there are tonnes of Meebo fans out there, so let us know how the new extension is working out for you in the comments.