SurplusMeter does precisely what we need it to: it watches your bandwidth traffic, keeping an eye on it like an eagle watching her brood. SurplusMeter gives users the ability to set a bandwidth limit, a start day of the month, and the connection type--PPP Modem or Network Card, for example.
SurplusMeter also calculates a daily allowance, and reconfigures that depending on how much you've actually used for the month. It lacks the not-quite-fancy graphs that many of its PC counterparts sport, but SurplusMeter also lacks something they have: an alarm or notification that would tell you when you were approaching your limit. We'd rather have the latter and suffer through the former, than have neither.
Fortunately, the interface is dead simple. All the options are presented in an uncluttered layout--you'll get exactly what you need from this app with a minimum of hassle. Handy progress bars give a slight visual flair to the days left in the month, the download megabyte count, and the total downloaded and uploaded. This will appeal to users suffering through bandwidth caps and those who just want to know how much they're downloading and uploading.