Android: If you need to keep a close eye on your talk time, data or texting, Stats is a free Android app that not only tracks your usage but sets limits so you don't get slapped with overages.
Stats catalogues incoming and outgoing calls, sent and received text messages and your mobile and Wi-Fi data usage in clean interface that has a surprising number of features underneath. You can set alerts, see what percentage of your plan you've used up and sync your tracking to your billing cycle by matching the automatic reset to the length of your cycle. In addition to being alerted when you're getting close to your monthly allotment, you can also set an alarm point where data transmissions will be cut off to protect you from overage charges.
Stats is free and works wherever Android does. You can download it by searching for "Stats" in the Android Market or scanning the QR code at right.
Have a preferred tool for monitoring your usage or an app you use on a different platform? Let's hear about it in the comments.