I've been using Mixpanel, but Woopra looks for powerful out of the box, at least for my needs. I've installed the tracking code and will play with it tomorrow.
I haven't tried Chartbeat or Reinvigorate.
OK, I'm on right now, and this kicks ass. This is exactly what I wanted at my last company. When you're dealing with relatively few (but very important) visitors, having this kind of real-time awareness is gold. I can't wait to try it tomorrow when more people are online.
chartbeat screenshot: http://dl.dropbox.com/u/186198/Screenshots/o0-4.png
reinvigorate screenshot: http://dl.dropbox.com/u/186198/Screenshots/mrva.png