Is there a good tool for monitoring network activity on Mac OS X?

Wireshark is widely acknowledged as one of the best network monitor tools available. Distributions are available for OS X.

If you prefer something more simplistic you can use iStatMenus to show incoming / outgoing network traffic speeds.

For the person who doesn't want to install anything you can also use tcpdump in Terminal.

tcpdump -i [interface, en0 is ethernet, en1 is wireless]

Also check out nettop, a little command line gem that ships with Mac OS.

Also look at MenuMeters. I tried iStat Menus & MenuMeters and preferred MM. Not saying one is better; just that there are a couple of choices.

A working version of MenuMeters for El Capitan can be found here.