T-Mobile USA's (TMUS +1%) Simple Starter plan provides unlimited talk/text and 500MB of 4G data for $40/month.
The carrier makes a point of noting the plan lacks the data overage fees charged by rivals. Assuming Simple Starter works like other T-Mobile plans, users will simply have their data speeds throttled after going over 500MB.
The plan is just the latest in a string of aggressive pricing moves from T-Mobile, moves that have triggered responses from AT&T and others. The company has already begun offering up to $650 in credit to get users to switch, and has rolled out relatively inexpensive smartphone upgrade and international roaming plans.