Truconnect
 

Truconnect TruConnect Mobile - Contract Free 3G Mobile Internet for Laptop and iPad looks an interesting option for contract-free data in the US (if it works and stays in business). Runs on Sprint. $5/mo to keep it active then 4c/MB ($40/GB).

Not cheap enough for everyday use, but not too bad for travel. Only works with a stick or mobile hotspot that you have to buy from them by mail within the US, which would make it mostly unsuitable for overseas visitors.

Despite the name, doesn't appear to have any connection with Tru / Trumobile. Since Truconnect's parent company is Telscape, it's odd they've picked a name very similar to another longer established company, plenty of room for confusion.