netmonitor is a small app that gives nice graphs, it can be adjusted to be displayed only in the menubar (no dock icon) and is a great little tool.
darkstat is a background deamon that gathers stats 24/7. Use it to view bandwidth per minute, hour, day or month and per host/protocol using a web-interface. Simple and effective.
ntop (also here) is a great app like 'top', but for networks. It's the app darkstat is a spin-off from, and it gives you a load of stats you can throw a stick at - also using a nice web-interface. Downside: it uses 75% of CPU resources here; I'm looking into it.
Use tcpflow if you want to see the data that goes in and out of your Mac in plain text. Great to cheat with flash games that submit scores to unknown hosts ;)
[Editor's note: I haven't tested any of these network monitoring apps.]

