It looks like following AT&T, T-Mobile, and Sprint the last major carrier to offer a month to month plan is Verizon Wireless.

Unlike the existing Verizon plan Inpulse which because of a daily access fee was essentially unusable, this is a true month to month offer that works with all Verizon Nationwide and data plans.  

The fine print is you either have to pay for an unsubsidized cell phone or use your own existing cell phone compatible to their CDMA network. The other catch is you can't avoid paying an activation fee, which can be as much as $30 per line. 

Via