BuddyPress.org

Andy, good advice, but when someone has disabled activities (as I have) , this doesn’t work.

slicktig, look here:

http://wpmudev.org/project/Sitewide-recent-comments

It’s a litte bit complicatet for you have to create the tables on your own and there is no widget, but it works. I changed the plugin a litte bit and it runs on http://www.tastentango.de now. If I would be able, I would make a widget of it but I have to solve some other problems first.

Hope it helps for the moment.