AT&T is upgrading two of its inexpensive pre-paid GoPhone plans today, but they come with some serious trade-offs. The first plan, with unlimited data for $60 per month, only offers 3Mbps of data, and the speed will drop even further after you've used 22GB of data. The second plan comes in at a budget-friendly $45 for 6GB of high-speed data, but you'll get throttled down to 128K when you exceed your cap. Both plans require an AutoPay option, but you won't have to sign a contract or undergo a credit check.
AT&T has been refreshing and upgrading its pre-paid phone plans for a while now (including a similarly structured plan for non-prepaying customers). Of the current offers, the more expensive one gets you unlimited talk, texting and data, but you'll be limited to the slower speeds -- almost half of AT&T's regular LTE throughput, as reported by the Open Signal project. Sure, unlimited data sounds great, but you'll be also be limited to "standard" definition video at 480p and there's no mention of using your phone as a hotspot.
If you need a low-cost option for your phone, one of these plans might be worth considering. Still, the low data cap and throttled speeds won't let you do much, making for a less complete experience.