As a software engineer with a particular interest in networking, Wireshark is definitely my #1 tool. If you're interested in learning more about networking, or how your applications REALLY behave on the network, Wireshark is the tool for you. It's every bit as full-featured as commercial network analysis products, and it has a vibrant community of users. There's a wealth of good "getting started" information on the Wireshark web site, much of which is linked within the application itself. Have I mentioned that this is my #1 tool?
Now, it isn't perfect--no network analyzer is--so you can't always take its interpretation as literal truth. Nonetheless, I find that it delivers everything I need and more. I use it not only for troubleshooting and debugging, but also for teaching and reference purposes.
The newest release, 1.6.3, is available at the link below. Wireshark binaries are available for Windows (32- and 64-bit), Windows U3 and PortableApps, OS X 10.5 (Intel & PPC), and OS X 10.6 for 64-bit Intel. Most Linux distrubutions deliver their own packages for Wireshark, but the source code is available for download if you want to roll your own install.
Get it. Use it. Learn it.