Australian ISPs generally set download quotas for their ISP plans.
(some like BigPond also set upload quotas as well!) 
The main rationale of this is mainly due to the fact most traffic is international traffic (US, Europe, Asia, etc), and as such ISPs need to purchase international bandwidth from private companies. These costs need to be limited and are therefore passed down to the consumer.
For users that go over their allocated and paid quota there normally are some really nasty side-effects:
- Rate limiting to ~64kbps. Making the connection only a slightly more bareable than dial-up.

- Excess charges, which can quickly add up on some plans.

As such, Aussie Internet users have trained themselves to avoid such pain; by always keeping an eye on their Internet Usage. ISPs provide a Usage Meter normally found on their website. However this can be tedious and means every time you need to check your usage you need to login to the ISP website; that's if you remember!

Unfortunately, with large speeds under xDSL available these days, it's quite easy to chew through your usage quite quickly.
An easy way to avoid such problems is to install a Firefox extension called
NetUsage. As the name implies, it keeps track of your Internet Usage.

Unlike other tools that track Internet Usage, it doesn't rely on SNMP from your router or count bytes in your browser, it actually reads your ISP usage meter and parses the information into a nice graphical representation.
This is accomplished either by screen-scraping the ISP usage website or via an XML usage data feed (preferred); if provided by your ISP - many do these days).
Screenshot 1: NetUsage running in Firefox under Windows.
It's quite clever too, and will cater for plans that have peak/off-peak periods. One feature I find useful is that at a glance I can tell if I'm over quota for the time period in the month.
A quick hover over the widget and I can actually see a great breakdown of the usage, how much per day I'm able to download until the end of the period (and remain under quota).


Screenshot 2: Hovering the over graph to obtain detailed statistics.
The application is well supported, and support is generally offered via a
Whirlpool forum thread.
^critter the author responds quickly and bugs and quirks are quickly jumped on.
If you're not in Australia, don't fret!
NetUsage does support some non-Australian ISPs.If your ISP isn't currently covered you can
provide the developer details of your ISPs XML usage data feed and he'll happily add it.

Better yet, it's cross platform and will run under Mac, Windows and Linux flawlessly.
Current releases also support the
latest Firefox 3 betas (as well as the current Firefox 2 of course).

Configuration couldn't be simpler! Right click on the graph, then select the preferences menu option.
As you can see there is a tonne of Australian ISPs catered for our of the box.
Screenshot 3: Configuration screen for NetUsage.
(Click image to view a larger image).So if you want to keep on top of your usage, give
NetUsage a go!

Those that want to get their hands dirty with the code, it's available via a
SVN repository at:
http://netusage.iau5.com/svn/
