Phone: Samsung Galaxy Light T-399.
Purchased for $50 through a slickdeal involving Walmart (phone sells new for $150). It's a T-Mobile branded phone, so it was locked to them. After a cheap unlock code I can now use it with my AT&T SIM. Galaxy Lights are all the same as far as GSM/LTE bands go, and work on T-Mobile and AT&T's LTE. The MetroPCS variant already has a KitKat ROM available, my T version doesn't yet. It's fairly fast for a budget phone, and has the same screen size as an iPhone 4/5.
Plan: I bought an iPad 3G model back in 2009 when they first came out, and as such I was able to purchase the $29.99 unlimited plan that went with it.
This plan was only sold for 3-4 months so at this point in time it's extremely rare and expensive on eBay. It's the only AT&T plan that actually has true unlimited without throttling. Various theories exist as to why there are no restrictions imposed on it, but the general consensus is that so few people were able to buy this before it was discontinued that AT&T didn't care what people did with it. More important than being unlimited is it's also portable, I can move the SIM to anything I want: hotspot, USB modem, smartphones. So after I figured this out a few years ago I've used it in phones exclusively. Tethering also works so I don't have internet service at home. Most I've ever used in one month is ~50 GB. The only downside is this is DATA ONLY, so I text through Google Voice and have a dumbphone I carry sometimes when the need for calls arises. The best plan for unlimited would be one of those Verizon unlimited ones, due to FCC regulations it's also portable and unthrottled, plus has voice service.
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^^^ Lycamobile is the best SIM to have if you need a glovebox phone. Refills are $10 minimum, and you only need one billable event (read: phone call or text) every 90 days to keep service active. So if you never use it, at 2 cents per minute $10 will last forever. You will need an unlocked phone in some cases to use Lycamobile because the early SIMs they issued use a different MCC (mobile carrier code) than T-Mobile does (although it uses their network), so T branded phones won't work if locked.
New SIMs from Lyca use a different plan that costs more, but I think the expiry is the same. They also use T-Mobile's MCC so locked phones will function with it.
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Data-only:
If you just need data, FreedomPop is the way to go. Practically free devices, and free 500 MB of WiMAX/LTE service through Sprint per month, which can be doubled by adding a bunch of "freedom friends" e-mail addresses. You can stack some of their sub-$5 addons like 3G network access and data rollover.
I have two FP devices, a Novatel MiFi 500, and a Franklin U600 USb modem. The MiFi has good battery life and is future-proof because it supports LTE on all of Sprints planned deployment bands (800, 1900, 2500) in addition to dual band CDMA/3G.
It also supports USB tethering to your PC so you can use and charge it at the same time. The only downside is there are zero external antenna ports so if signal sucks where you are it won't be usable. FP does offer a MiFi device with antenna ports however it's an older model which only has LTE support on 1900.
The Franklin U600 is a WiMAX and CDMA device, but does have antenna ports.
They're giving these WiMAX devices away for free or next to nothing because Sprint is shutting off WiMAX this year so after that only the CDMA/EVDO portion will function.