which tools do you use, & why?

tried Charles, but only 30 day license is free ($50/node after that).
fiddler is nice, but it's like drinking from the hose

my goal is to find out why we're having collisions b/t session cookies that we've demangled in our app server (WAS 6.1), but does not appear to have been pushed. hoping by looking at individual packets/sessions we can where FileNet stomps on our web app