Lifehacker's found a very useful Firefox extension for our Verizon and T-Mobile readers. The Minutes Used extension adds a little note at the bottom of your browser, in your status bar, to display how many minutes you've used and how many minutes you have total for the month. You can mouse over it to get more info such as your projected amount of minutes, last statement date, and peak/offpeak/most recent calls (for Verizon).
It's great for keeping an eye on your kids to make sure they don't go over minutes, causing you to confiscate their phones and shouts of "I hate you dad! I'm never going to grow up to be like you, EVER!" to echo through your house.