Mobile broadband is very common now that smart phones such as the iPhone and various Android devices are popular.
Data plans used to be unlimited, until smartphones made them say "whoa! We never imagined five years ago you'd ever use so much data!"
Except for those lucky enough to retain one of those old unlimited plans, 2GB is the most common limit, with 200 MB being the economy option, and 5+ GB available as a premium option.
Mobile wifi hotspot (wifi tethering) is available as an extra cost option on capable smart phones, or sometimes free if you hack your phone or use certain models with certain carriers (such as the T-Mobile G2, aka HTC Desire-Z).
Options vary by carrier, of course, but those are common examples.
Verizon Wireless is USA carrier considered the farthest ahead in the game as far as deploying the latest 4G LTE network technology. They also have the farthest-reaching 3G network based on a CDMA technology called EV-DO that's capable of about 2 Mbps max. Their 4G network gives me about 8-20 Mbps download on my HTC Thunderbolt.
Verizon states they will upgrade their entire network from 3G to 4G by the end of 2013. Based on their prior track record with 3G, and the current pace of the 4G roll out, I have no reason to doubt that.