Republic Wireless is offering an interesting new product: SIM cards that come pre-loaded with either 30 or 90 days of (technically) unlimited data. The new cards don't require activation, an account, or even a payment method after purchase. There are a few caveats, though.
There's always an asterisk when you're talking about unlimited data, and it's no different here. The cards do provide as much data as you want, but only a portion of it is at usable speeds: for the 30-day card, the first 20 gigabytes are at 4G speeds, and the 90-day card is allotted 20 gigs per 30 days (so 60 total, but divided into thirds). After you hit the cards' limits, any further data used will be 2G. Roaming also isn't included, so don't go buying one of these cards to take overseas.
Strangely, the cards are activated when they're delivered, not when you pop them in your device, which limits their utility even further. But if you have a device that only needs data, these are a pretty solid deal, particularly the 90-day variant. They're not reloadable, though, so once your card expires, you'll need to get a new one if you want to continue getting service.