GiffGaff, a subsidiary of O2, has a £10 goody bag offer which comes with 250 minutes, unlimited UK texts and unlimited mobile internet for one month, which is by far the cheapest such offer. In comparison, Three does 300 minutes, 3000 texts and unlimited data for £15 on a one month contract.
The SIM can be used in any O2 and Tesco Mobile phones. Giffgaff says that the "unlimited" bit of the contract is only valid for personal mobile use and not for commercial and automated use. You also get free calls to giffgaff users. Calls to free phone numbers are free while calls to other UK mobiles, landlines and call forwarding are 10p per minute after the inclusive minutes in the goodybag.
In theory, nothing prevents users from using the SIM card in an Android smartphone (from Android 2.2 onwards) and tether from it to multiple devices. Or use it in a 3G enabled tablet (like the original Samsung Galaxy Tab).
That said, if you don't need any minutes or texts, surely T-Mobile's 3G SIM hack which costs only £10 credit for a year and give you 500MB data per month (with free unlimited email and web browsing) may be better for some.
Source : Giffgaff