Wireshark (or Tshark) is probably the defacto standard traffic inspection tool. It is unobtrusive and works without fiddling with port redirecting and proxying. It is very generic, though, as does not (AFAIK) provide any tooling specifically to monitor web service traffic - it's all tcp/ip and http.
You have probably already looked at tcpmon but I don't know of any other tool that does the sit-in-between thing.