The one I'm familiar with is iPass, which bundles and resells access including WiFi, wired Ethernet and DSL, mobile broadband, and even dial-up. iPass doesn't sell directly, they go through resellers, and within the past year, they added VPN and other security and connection management services.
As of a year ago, when I was trying the iPass network with both 3G and WiFi -- iPass' and its resellers' sites are far from helpful in terms of current offerings and pricing -- you paid month-to-month, and would only be charged for use of mobile broadband access during months when you used it.
Conclusions and suggestions
Domestically, if you've got a smartphone, "tethering" (using your smartphone as a mobile broadband modem), may be your most cost-effective solution. Especially if tethering is a separate cost (like AT&T Wireless, $20/month), and if there's no per-transaction cost to add/remove that service (AT&T says there isn't).
If you're going to be in another country for a few days to a few weeks, a local prepaid mobile broadband offering, or one that will work in that country, may be your best bet. It may not be the cheapest per day or megabit, but the zero-commitment counts for a lot, especially if you can use VoIP (Skype, GoogleVoice, etc.) to avoid or minimize those expensive international cell phone calls.
Got any better ideas? Let me know!