In the last few years, a ton of new MNVOs have come out offering very cheap cellphone service. An MNVO is an independent company that uses the big 3 towers and signals (Verizon / AT&T / Tmobile-Sprint).
For example, you can get unlimited service on a family plan with Cricket including 15gb of hotspot usage for $60 per month including taxes. And they use AT&T towers, so if you have more expensive AT&T service, you'll probably save a lot by switching.
Here is a great article comparing MNVO plans - I'm not positive it is current though. (scroll down to the chart)
Note - most MNVOs do not offer phones for sale. To switch to them you will need an unlocked phone.
If you have been with one of the big 3 for a few years, if nothing else, go onto their website and compare your current plan / cost to what they are offering now. Prices have really dropped in the last few years!
Finally, figure out if you really need unlimited. Calls and texts are nearly always unlimited. But if you are on wifi at home and only use 2gb of cellular data, that is available from cricket for $30 per month. If you can switch from a $60 plan to a $30 plan, that is a $360 per year savings!