I will say all the charity support I've received this last year was from friends and family - but last time I needed help I got some from Salvation Army.
OpenFire is the tool we used at the last shop I worked at - for exactly this. (it's a java-based server and will run on many server types including but hardly restricted to most Linux distros and Windows) They've got some great commercial tools as well.
for something requiring more technical workings of the software - jabber2 and ejabberd both are superior - but take more configuration.
For clients - there's the Spark client also from IGN software - which works well enough. otherwise a wide variety of opensource clients support Jabber/XMPP. You can firewall out the ports externally to lock people into being able to only sign into the local net as well, easily enough.
note: I currently do not work for anyone so I do not speak for any agency.
If I have not seen so far it is because I stood in giant's footsteps.