ON A CONTINUAL DAY TO DAY BASIS
Generally we use an external application NTOP for this, it works great for eveyday use (figuring who's the top talkers, etc) as well as baselining and knowing when and where to think of vlan'ing.
You just need a switch with a monitor/mirror/span port and a linux box (with ntop). After a little bit of setting up, you are good to go.
AD HOC, UGLY IN AND OUT (Command Line) WAY
or you can do it the Gosh awful way which I also do (on the subnet level, not by host).
If your network is small enough, you can configure outbound permit ACL statements for each ip and see which one generates the most hits. Performance may suffer, but if you just want to take a snapshot over lets just say a week or so, you may have your answer. I use this on our routers on process switched interfaces, but should work with PIX.