You need to have a Nectar card online account for this one.
Nectar have an offer on at the moment where when you buy an O2 PAYG Mobile Broadband Dongle via their website, you receive 3,750 Nectar points! However, O2 also have a special offer this weekend where the Dongle only costs £10 this weekend.
So basically a £10 spend will return points equivalent to an £18.75 spend in either Argos or Sainsbury's - a PROFIT of £8.75 and as a bonus, a PAYG Mobile Broadband Dongle!
Alternatively, spend your 3,750 points on any of the other treats and offers on the Nectar website.
It appears there is no compulsory top-up required to qualify for points unlike Quidco, so you don't need to use the Dongle - you can do what you like with it.