Here is a link to the myBB plugins page:
https://mods.mybb.com/mods
But, none are guaranteed to work, and I have installed a few that didn't. There is no instant messaging plugin available that I know of, and it would require quite a bit of effort to even develop one.
Also, there are some plugins that are not compatable with our forum, even though they are myBB plugins, because I have made a number of customizations and modifications to the scripting and templating. It just depends on what files the plugins are accessing and what files they are modifying.
For instance, we cannot use any plugin that modifies the profile template, postbit, reputation, or a number of other templates because I have made modifications to each. We also cannot use any plugin that accesses or modifies the member or reputation scripts, again becasue of modification I made.
And there are many others too, I just don't remember which ones offhand, but when I look at the code in plugins and see it wants to aaccess or modify anything that I have changed, I sure as hell remembe at that time.
And, I've got a duplicate of this forum setup on another domain that I use for installing plugins, modifying files or templates or anything and every thing else I want to mess with, so I can test it first and make sure nothing going to get screwed up, before incorporating it here, just in case.