Lots of Android users are on unmetered data plans – for example, the Sprint and Verizon networks in the US have unlimited plans (though the latter, only if you've been grandfathered in). But the rest of us has to live with only a certain amount of data each month, and the overage charges can be pretty punitive.
Fortunately, it's fairly easy to make sure you're living within your means, data-wise. Ice Cream Sandwich gives you a built-in way to keep tabs on your data. Just head over to the "Data usage" section of settings. In addition to showing you how much mobile data each app has used, it'll also let you set a warning when your data usage approaches a certain limit and a hard stop when it reaches a particular threshold. Let's say you're on a standard 2GB/month plan. You could set a warning when you reach 1.5GB, and a limit on data when you get to 1.9GB.
           For Froyo, Gingerbread, and Honeycomb, try the free Onavo app. It actually has a few more features than Ice Cream Sandwich's data monitor, including the ability to set limits for individual apps and/or restrict them to Wi-Fi only.