I'm looking for a program that I can install on a Mac that will tell me how many bytes I download each day, and store that info in such a way that I could later view the results.
Limiting by ports (80, 443, 21, 22) would be awesome as well.
Does such a thing exist?
Ok, this isn't a complete package or anything, but netstat will show you bytes transmitted on an interface
netstat -ib
you can record this somewhere every day. "man netstat" for more info.
You could run a local web proxy, then analyze the logs.
It's simple to download and install SquidMan, which is a point and click way of enabling the squid web proxy on your system.
You will have to configure your machine to go through the proxy on localhost instead of directly to the website. You can find this in advanced tab of your network interface in System Preferences -> Network
Then, you can analyze the logs and see not only how much you've downloaded, but also what and when.
Another tool for this is ethernal but it hasn't been updated for a while.
I just came across these two apps, Net Monitor and Net Monitor Sidekick, the first to do what you describe (with a calculator to determine throughput over a date range), the second to track traffic by host. After trying to use ntop, which is horrible to set up, hideous to use, and very, very limited, I'm thrilled =)