Last Day of offer!
O2 dongle is currently half price for £10.
(24 hours) - For £2 for 500MB + Unlimited Wi-Fi
(7 days) - For £7.50 for 1GB + Unlimited Wi-Fi
(30 days) - For £15 for 3GB + Unlimited Wi-Fi
Nectar are also offering 1250 points (£6.75 to spend in argos/sainsburys/homebase) making the dongle £3.25. Link to Nectar offer in second post.
O2 decline orders if you order more than three so if you ordered when it was last on offer you won't be able to buy any more.
Link to last deal when nectar was offering 3750 points...http://www.hotukdeals.com/item/590511/o2-payg-mobile-broadband-dongle-for
Most people haven't got their points yet but some have, and the rest should have them soon enough.
Quidco also offer £5 but you have to topup with £15.
EDIT: Most have now had their points, many without having to chase them up, although they took a bit longer than the 31 days they say in the t&c.