Bandwidth caps are all the rage these days, in North America and around the world. We're talking about the notorious ceilings on how much broadband data you can use before your ISP starts charging you extra, or slowing down your connection, or shaping your traffic use (eg, blocking your access to certain applications).
AT&T's shiny new caps started on May 2: 150GB a month for DSL and 250GB for UVerse. AT&T customers will get a $10 charge if they cross the line, with that ding repeated for every 50GB thereafter. Comcast set a 250GB cap in 2008. Canadian ISPs have had them for a while, too.
Ars readers sometimes write in asking us how they can monitor their bandwidth use to avoid extra charges. Here are some links and pictures from major ISPs, which are broadly representative of the tools used by most Internet providers.