Wireshark is a network protocol analyzer. Some may recognize it by its former name, Ethereal. It's free (and open source), runs on multiple platforms (including Windows and Linux), and actively developed. For those doing VOIP installations or troubleshooting existing installations, the latest release has some very handy VOIP specific support.
It will create visuals representing captured SIP and associated RTP connections. You can drill down by clicking on specific spots on the graph to pull up the associated packet(s). You can generate reports (as well as graph) jitter, bandwidth usage, etc. Various ways of displaying the data to get a better idea of what's really going on.
The screen captures at the beginning of this post are from Wireshark. They show a graph of a VOIP (SIP) call (and a half) between two Snom SIP phones attached to an Asterisk-based PBX (the green/blue/purple image). And an analysis of the associated RTP session (including packet loss, jitter, delay). WS can even playback captured VOIP calls (at least if using PCM/G.711/ulaw).