If you've ever needed to get intimately familiar with a protocol, picking it apart with a packet analyzer is the way to do it. Unfortunately, we often don't have direct or immediate access to the traffic flow we wish to observe. Fortunately there are a number of sources around the web which provide various packet captures pre-assembled and free for download.

One of my favorite such sources is the Sample Captures page of the Wireshark Wiki.

While there is certainly a wealth of captures, the page is admittedly a bit chaotic (wikis aren't the ideal catalyst for file management). Rather than relying on the hastily arranged categories, I recommend doing a full-page text search for items of interest. Sharing is encouraged, and particular requests can be read/fulfilled toward the bottom of the page.

A word of warning: keep in mind that anyone can contribute captures. Given the history of vulnerabilities in some of Wireshark's protocol descriptors, proceed with caution and never open untrusted captures with elevated privileges.