Unsure if this is one of the good ones or not (is it the 3 month unlimited, or just 30 day?. I know it'll be limited availability, but thought I'd share.
"Enjoy an unlimited data allowance with this T-Mobile pay as you go mobile broadband USB. Simply plug into your PC/laptop via the USB port and you could be online within a matter of minutes. This USB can be used as a memory stick too. Top up from £2 for a day, £7 for a week and £15 for a whole month."