Stats is a great Android app for tracking phone usage
Most carriers don't just offer one cellphone package; you usually get a wide variety of options – how many text messages per month, how much data traffic, how many minutes, etc.
Up until today, I would usually lean towards the "unlimited" options, just because it made me feel safer. After all, nobody likes to find out at the end of the month that they were charged extra for going over the limit. And even if your provider sends you a "usage warning" SMS before you go over the limit, that's no fun.
Stats is a free, comprehensive Android app that seeks to help out with exactly this situation. This little gem quietly collects all of the stats concerning your phone usage: data in and out over 3G and over Wi-Fi (separately), SMSes sent and received, minutes called and received – everything.
You can also set up limits, and the app notifies you when you hit that limit. It's very useful for benchmarking, too. For example, if you're curious to know how much data one minute of Skype consumes, just check Stats first, make a call, and then check Stats again.
Stats is completely free, and it has no in-program banners or other nags. I highly recommend it.